Mass.) Marine Biological Laboratory (Woods Hole.

Biological lectures delivered at the Marine biological laboratory of Wood's Hole ... 1890-[1899] (Volume 7) online

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cell generation as a rule. Moreover, these are the first evidences
of bilaterality. In no case have distinctly bilateral divisions
been found at an earlier stage. Wilson regarded this fact as
one of the most striking features of the cleavage. In Crepidula
the change is not so abrupt, according to Conklin, though it
appears in almost exactly the same cell generation as in Nereis,
at least in certain parts of the egg. Wilson believed the appear-
ance of bilaterality was due to the reduction of the " left pos-
terior macromere " to the same size as its fellows as the result
of the formation of the first somatoblast and the mesoblast.
Conklin has shown, however, that this rule does not hold good



254 BIOLOGICAL LECTURES,

in the case of Crepidula and many other gasteropods, since here
the " left posterior macromere is not appreciably larger than the
right, and in some (e.g., Umbrella, Heymons, '93) it is smaller,
and . . . the mesoblast (4^) is only one member of a quartette
which is separated in a left spiral from the macromeres, each
of the other members being quite as large, or even larger than
the cell 4^/." And finally, after consideration of the manner in
which bilateral cleavages arise, he says : " The conclusion, there-
fore, is unmistakable that bilaterality first appears in processes
which lead to the formation of the trunk and the elongation of
the future animal, while the primitive radial symmetry of the
anterior quadrants is correlated with the fact that these quad-
rants give rise largely to larval organs, most of which bear
traces of radial symmetry." And again it must be said of this
conclusion, as Conklin said of Wilson's, that it is not applicable
here, or applicable only in modified form, for the anterior quad-
rants show a high degree of bilaterality in many forms.

I believe the facts as known thus far indicate that the func-
tion of bilateral cleavage is the symmetrical distribution of the
material for the formation of the adult or larva. It is evident
that in the adaptation of means to end symmetrical divisions
would be useless at a stage when the material was not in a posi-
tion to be distributed. Suppose the radial divisions forming the
cross (Fig. 7) appeared before the cross cells were surrounded
by a complete ring of cells. The formation of the cross would
not result in forcing the ectodermal cap down over the egg,
but would simply produce four lines of cells extending outward
from the anterior pole as far as or beyond the other ectoderm
cells. Again, suppose the first somatoblast 2d were to divide
bilaterally at its first division or its second : the growth of the
somatic plate in the direction which is to result in its concres-
cence would not be begun ; but, as it is, two cells are formed,
lying on either side of, and posterior to, the stem cell (Fig. 9),
and then a cell is given off anteriorly in the median line (Fig.
10), and thus the whole mass is forced somewhat posteriorly
and the symmetrical distribution of the material is begun.
Then in succeeding divisions of the stem cells (Fig. n-i/)
how exactly the relation between the parts is preserved, every



THE SPIRAL TYPE OF CLEAVAGE.



255



division balancing others and aiding in the accomplishment of
the " desired" end, viz., the concrescence of the plate and the
formation of the " growing tip " ! Moreover, the posterior move-
ment of the descendants of the first somatoblast which is ini-
tiated by the third division the first symmetrical division
begins the covering over of the mesoblast. The mesoblast
divides bilaterally at its first division, for it is formed in such a
position that it must be covered over by the somatic plate and
thus forced into the interior of the egg, and its fate is to fur-
nish the double " Anlage " for the mesoderm bands. I think
these examples are sufficient to show how exactly the appear-
ance of the bilateral cleavages is timed for the accomplishment
of the purpose of development in the most perfect manner.
And, to my mind at least, they furnish ample explanation of
the fact that the bilateral cleavage does not appear in the
earlier stages.

It may be urged as an objection to this view that in various
forms (e.g., ascidians) the first cleavages are bilateral, and the
distribution of the material is accomplished equally well. This,
of course, is perfectly true, but there is no evidence that the
bilateral form of cleavage has displaced a spiral form in this
case. In the annelids and mollusks the spiral form of cleavage
is undoubtedly older than the bilateral form, and has been in
part displaced by it, the change occurring first in later stages,
and proceeding to the earlier. The point made here is simply
that there is no adequate cause for the displacement of the
spiral by the bilateral form of cleavage in stages earlier than
those at which the latter now appears, unless of course the
spiral period could be eliminated at one blow, as it were, and a
bilateral cleavage could begin at once. This may perhaps be
a more complete modification, but there are no indications dis-
cernible of its appearance, unless, indeed, the cleavage of such
forms as Teredo and Cyclas is to be regarded as such. The
spiral form of cleavage seems to serve perfectly in plotting out
the material of the egg in these forms.

In general, the direction of each cell division in morphogenetic
cleavage plays a perfectly definite role in the accomplishment of
the morphogenesis of the species concerned. Bilaterally symmet-



256



BIOLOGICAL LECTURES.



rical cleavage is necessary for the symmetrical distribution of
the precociously segregated material, so that it is to be expected
that the axes of symmetry in the cleaving egg should correspond
with the axes of symmetry in tJie adult. It is probable, however,
that the establishment of these axes is significant as an incident
connected with the mechanics of this form of cleavage, rather tJian
as indicating differentiation.

The time of division in the different blastomeres is also
a factor in the result attained. As mentioned above, the time
at which the spiral form of cleavage ends is closely connected
with the process of gastrulation, etc. All through the bilateral
period of cleavage the divisions seem to occur at the right time
to serve as a factor in morphogenesis. In the case of symmet-
rical divisions on the two sides of the egg this fact is especially
noticeable, and the best illustration is found in the later divisions
of the somatic plate (Figs. 11-17). The symmetrical divisions
occur at the same time, or nearly, although they may be almost
on opposite sides of the egg. Sometimes one side is slightly
ahead of the other, but in any case the variation is not great,
and the arrangement of the material is the same in every case.
In general, it may be stated that the larger cells divide more
rapidly than the smaller, but this rule does not hold good in

all cases, and does not apply of
course to yolk-laden cells. In-
deed, as Conklin ('97) and Lillie
('95) have suggested, the rapidity
of division is apparently regulated
in some cases by the time at which
the portion of material concerned
is to become functional, but
neither does this rule apply to all
cases. For instance, the primary
trochoblasts in Arenicola are rel-

FIG. 23. Clymenella. Small size of cross cells <- ~U. Crrm l1p r f-V,o n i n A>ni<hJii
and large size of primary trochoblasts (after atlVely L Amptll-

Mead )- trite and Clymenella (cf. Figs. 7

and 23), and become ciliated at a much later stage. Yet in
the two forms the trochoblasts pass through all of their divi-
sions with equal rapidity, i.e., relatively to the other blasto-




THE SPIRAL TYPE OF CLEAVAGE.



257




meres. The early division of these cells in Arenicola, where
the cells do not become ciliated for a long time, must, I think,
be considered as without significance for the prototroch itself,
but as directly connected with the overgrowth of the entomeres
by the ectomeres. In Sternaspis the divisions of the cells
which correspond to the trochoblasts occur at almost exactly
the same time relatively to
the other cells, but here there
is no prototroch, and these
cells apparently form merely
a part of the ectoderm.

The mesoblast in Arenicola,
though larger than most other
cells in the egg, divides very
slowly. This is not wholly
because of the fact that it
becomes differentiated only
at a late stage, for the ante-
rior ends of the mesoblast FlG - **>-

ing the blastoccele. Optical section.

bands form muscles in the

trochophore. But there is no room for the products of
its division in the blastoccele in early stages (Fig. 24). If it
divided rapidly, invagination of the entomeres would be impos-
sible, unless the ectoderm were stretched to a considerable
degree. In cases where the mesoblast is smaller, the rapidity
of division may be more closely connected with the time at
which it becomes functional as mesoblast.

The size of the various cells is another factor. It is appar-
ently correlated with several features of the process of morpho-
genesis, viz., the final fate of the cells in question, the stage at
which the material is used, and the relation to other cells in
the complex. Lillie and Conklin have called attention to the
first two of these factors, but the third seems fully as important
as either of the other two. The size of the various cells is
an essential factor in the accomplishment of the processes of
growth. Examination of the growth of the somatic plate gives
the impression that not only is the direction of division
adapted to the form of growth, but the size of each cell also.



258 BIOLOGICAL LECTURES.

A bit of material is given off at one point, serving to fill up a
chink, as it were; a large cell appears at another point, etc., and
every cell in the complex aids in the postero-lateral growth
and concrescence of the plate, not simply because it was
formed in a certain position, but because it is of a certain
size. Some cells, like the turret cells of Crepidula, are small
when formed, but later increase greatly in size. In these cases
it seems probable that the appearance of a large cell at the
time when these divisions occur would interfere with the dis-
tribution of the ectodermal material. The later increase in
size may be connected with the role which the cell itself is

to play, or may be another
means of aiding in the distribu-
tion of the material.

But in many cases a great
difference in size is found among
the cells at the very beginning
of cleavage, as evidenced by
the extremely unequal division
in many forms (Fig. 25). This
difference continues to be ex-
pressed at various points during
Unequal two-ceii the spiral period, notably in the
formation of the first somato-

blast, 2d (Fig. 5), and the mesoblast, 4^, and, in a less degree,
in many other cells. These are the differences which have
especially attracted the attention of nearly all those who have
studied this form of cleavage. They have been quite generally
regarded as differential in significance. There is here undoubt-
edly an anticipation of, and provision for, the later stages of
cleavage, but, to me at least, it seems that, so far as morpho-
genesis is concerned, it is, at least usually, rather quantitative
than qualitative. Certain amounts, rather than certain kinds,
of material are stored up in certain cells just where they will
be in position to produce by coordinated action the " desired
result." Thus a still further " condensation " of the process
of development is effected. Cells which are to serve as centers
of distribution of material, e.g., the first somatoblast, may exceed




THE SPIRAL TYPE OF CLEAVAGE. 259

all other cells of the egg in size, as is the case in Arenicola.
It seems at least unnecessary to suppose, however, that this
cell contains an entirely different "organ-forming substance"
from that contained in, for instance, the four cells, products of
the dorsal intermediate girdle cell of the first quartette, which
pass through the dorsal gap in the prototroch, and lie just in
front of the somatic plate (Fig. 13), or from that contained
in the cells of the second and third quartettes which form the
ectodermal region about the mouth. Indeed, Dr. Treadwell
tells me that in Podarke a large portion of the dorsal ectoderm
of the trochophore is formed by the descendants of the dorsal
first quartette cell, which have passed through the gap in the
prototroch.

Of course, difference in size between yolk-bearing and non-
yolk-bearing cells is an expression of difference in constitution.

Notwithstanding the fact that the spiral form of cleavage
remains throughout the earlier stages of development, a form
of modification does occur which in a way anticipates the morpho-
genetic character of the later cleavage. The determinate char-
acter of the morphogenetic period establishes the fate of each
cell under normal conditions ; and, moreover, it renders possible
a further degree of modification in the cleavage, viz., " preco-
cious segregation." As the result of precocious segregation,
the blastomeres not only have a definite fate, but each contains
a definite amount of material, according to the part which it is
to play in the cleavage and irrespective of the yolk which it
may contain. This modification extends back to the earliest
stages of cleavage, but without altering the spiral form, not-
withstanding the great differences in pressure that may exist
between the blastomeres. It has become the custom to speak
of a protoblast as containing from its earliest appearance the
substance for the organs which are to be formed by its descend-
ants, and the term "differential cleavage" has been employed
to denote the divisions which give rise to the protoblasts. Pre-
cocious segregation is undoubtedly connected with differentia-
tion in so far as the perfectly exact distribution of the material
renders an earlier differentiation possible ; but, as regards the
separation of various organ-forming substances by the planes



260 BIOLOGICAL LECTURES.

of cleavage, it is very difficult for me, at least, to believe that
it occurs, especially as I cannot see that there is evidence to
warrant the conclusion. There are, of course, cases where cer-
tain substances have been found to pass always into definite
cells, e.g., the granules of the " Urvelarzellen " of Neritina
(Blochmann, ( 8l) and certain granules in Asplanchna (Jennings,
'96). Evidence is lacking, however, to show that these sub-
stances are necessary to the formation of the organs to which
the cells give rise. Conklin ('99, p. 18) admits that there is
abundant evidence to prove the absence of any necessary rela-
tion between cell formation and differentiation, but believes
that in certain cases the two processes are related. He distin-
guishes three categories of relation : (i) " Cell formation follow-
ing the lines of preceding differentiation, e.g., certain cleavages
of ctenophores, mollusks, and ascidians ; or (2) cell formation
and concomitant differentiation, e.g., many cleavages of turbel-
larians, nematodes, annelids, and mollusks ; or (3) differentia-
tion following the lines of preceding cell formation, e.g., many
cleavages in the eggs of annelids, mollusks, and probably many
other animals." The significance of these so-called relations
becomes apparent when we observe that every differentiation
which occurs in multicellular material must be related in one
of these three ways to cell division. In other words, cell divi-
sion and differentiation are just as independent of each other
here as elsewhere, but the process of differentiation is occur-
ring to a certain extent during a series of very conspicuous cell
divisions, i.e., the earlier ones. A comparison of the process
of formation of a single organ, the prototroch, in different ani-
mals seems to me to show how absolutely distinct cell formation
and differentiation are. The prototroch may consist of eight
or of sixteen cells, of twenty-five, of thirty, or of many ; the
series of divisions leading to its formation are almost always
different in different forms, and where they are not they are
similar simply because they are purely spiral cleavages ; it may
become functional almost immediately after the divisions are
completed, indeed in Capitella (Eisig, '98) divisions continue
after ciliation, or it may remain a long time before becoming
ciliated ; and, finally, cells perfectly equivalent to the " primary



THE SPIRAL TYPE OF CLEAVAGE. 261

trochoblasts " in origin may be formed without ever forming a
prototroch.

It is hardly necessary to remark that the differentiation of
an egg into protoplasmic and deutoplasmic portions, which so
often accompanies cleavage, undoubtedly has, in most cases at
least, a comparatively simple explanation.

Differentiation is then, I believe, " a function of position" (cf.
Driesch). Whether the material is contained within one cell or
many is a mere incident so far as the result is concerned. The
comparative study of the spiral type of cleavage leads irresistibly
to this conclusion.

The appearance of definite protoblasts in cleavage does not
necessarily imply that they contain a specific material which is
necessary for the formation of the organ in question. Proto-
blasts are, I think, to be regarded, in general, as centers of dis-
tribution of the material of the egg; and their formation is
probably due to a condensation in the process of development,
or a saving of energy, as Conklin suggested. As I understand
Conklin's position, however, he regards the saving of energy as
occurring in processes of differentiation merely, for, as he
remarks, " it is possible to see that in an organ which reaches
functional activity after a dozen divisions less energy has been
expended than in one which reaches this stage only after one
hundred divisions." This is undoubtedly true, but how does it
explain the formation of the whole trunk ectoderm from the
first somatoblast, which occurs in some annelids, or the forma-
tion of the mesoblasts, for the descendants of these cells do
not, for the most part, become "functional" in the definitive
sense for a long time, and then chiefly with reference to the
adult body and not the larval ? There is surely no reason for
believing that the number of cells in the organs of the adult is
less in those forms in which precocious segregation has occurred
than in others. Lillie ('95) has pointed out the fact that "almost
every detail of the cleavage of the ovum of Unto can be shown
to possess some differential significance." It is undoubtedly
significant as regards the distribution of the egg material, but
that is not necessarily differentiation. The material separated
as the result of precocious segregation may, I believe, be per-



262 BIOLOGICAL LECTURES.

fectly indifferent material, except as regards position, and the
true significance of the segregation may lie in the fact that a
certain amount of material is present, which is to be distributed
through the means of a series of perfectly definite cell divisions.
By " distribution " is meant the arrangement of the material in
such a manner that the various morphogenetic processes are
directly accomplished thereby, e.g., formation of the somatic
plate, gastrulation, which in Arenicola and some related forms
is probably accomplished very largely by the growth of the
somatic plate, formation of the mesoderm bands, beginning
elongation of the larva, etc. The important point in this view
lies, I believe, in the conclusion that these results are accom-
plished, at least very largely, directly by the energy of cell divi-
sion itself. Given a certain degree of cohesive power in the
protoplasmic material, a certain degree of surface tension, and
a certain definite series of cell divisions and the result must
always be the same, and the same cells will always occupy the
same relative positions at corresponding stages and will form
the same parts of the whole, i.e., will have the same fate under
normal conditions. This would be true even if the segregated
material should remain totipotent long after its segregation.

Since the fate of the various cells is definitely established
and can be foretold, it is possible that differentiation might
begin at an earlier stage than otherwise, but there is no reason
to suppose that the process of cell division is connected with it
even here. To take an example : why should the two lateral
cells of the paratroch on each side in Arenicola pass through
one more division than the dorsal cells before they form the
paratroch (Fig. 1 5) ? If cell division is connected in these
eggs with differentiation, it must be either that one of these
divisions is "non-differential," according to Conklin's distinc-
tion, or else it is necessary to assume that the lateral cells
are more highly differentiated than the dorsal cells. As
regards the first assumption, there seems to be no definite cri-
terion of differential divisions, so that no decision is possible.
As regards the second, there is no reason to believe that it is
true. Taking up the matter from a different point of view, viz.,
the relation of the constituent parts with regard to the result



THE SPIRAL TYPE OF CLEAVAGE.



263




to be accomplished, an explanation appears possible. This divi-
sion is a necessary step in the concrescence of the somatic plate.
It does not bring " paratrochal material" into the proper posi-
tion or into the proper degree of differentiation, but it does aid
in accomplishing the end toward which each of the symmetrical
divisions in the somatic plate contributes its part. In another
case, e.g., Ampkitrite, where some of the preceding divisions
are different and the material is
somewhat differently grouped,
no such division occurs, and
the paratroch consists of four
cells instead of six.

The extreme precocious seg-
regation, which extends back
even to the first cleavage and
in some forms (Arenicola) is
indicated even before cleavage
by the shape of the egg (Figs.
26, 27), is rendered possible by
the fact that the whole cleav-
age is strictly determinate in
character. A determinate form
of cleavage is, as I believe,
the product of a considera-
ble degree of modification in
development in the direction
of condensation. Thus the ap-
pearance of precocious segre-
gation in the earliest stages must indicate a still further
departure from the original primitive forms of cleavage.

Precocious segregation reaches its highest expression at
present in the oligochaetes and the leeches, especially in the
latter, where the development is very largely teloblastic, the
material for almost the whole body being segregated into a few
cells which form the germ bands. Here is not only segrega-
tion, but regulated cell division in the highest degree.

Teloblastic development must undoubtedly be regarded as
the last term in a long series of modifications, all leading to a



FIG. 26. Arenicola. Egg before division,
seen from upper pole.




FIG. 27. Arenicola. Egg before division,
seen from side.



264 BIOLOGICAL LECTURES.

shortening or condensation of the process of development,
although this, as Conklin has remarked, does not necessarily
signify a shortening of the time of development. I have at-
tempted in the preceding pages to follow the various steps of
the process, as I believe they must have occurred historically.

It appears probable that a large amount of yolk affords a
condition favorable to the teloblastic form of development, for
it renders possible the very early localization of the embryo
upon certain portions of the egg. In the annelids and mollusks
the amount of yolk in the egg is not sufficient to render any
such localization of material necessary in the interests of con-
densation of development, and a partial localization of material
at various points is the most favorable arrangement for rapid
development.

In any form of cleavage which is morphogenetic, it is incor-
rect to regard certain cells as becoming functional at certain
times. Each cell is functional at every stage of the cleavage
and directly connected with morphogenesis. Since this is the
case, it is necessary in seeking for the explanation of its vari-
ous qualities, such as structure, size, direction, and time of
division, etc., to consider, not simply the part which it is to
play in the adult organism or in the larva, but the part which
it plays at the moment under consideration and at each moment
before and after it. Each stage of development is the result


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Online LibraryMass.) Marine Biological Laboratory (Woods HoleBiological lectures delivered at the Marine biological laboratory of Wood's Hole ... 1890-[1899] (Volume 7) → online text (page 23 of 25)