Harvard University. Museum of Comparative Zoology..

Contributions from the Zoölogical Laboratory of the Museum of ..., Issues 21-30 online

. (page 42 of 57)
Online LibraryHarvard University. Museum of Comparative Zoology.Contributions from the Zoölogical Laboratory of the Museum of ..., Issues 21-30 → online text (page 42 of 57)
Font size
QR-code for this ebook

part) of the polypide (Plate III, Figs. 22-25). Moreover, cross sections
of the anal part of the bud show the inner layer passing directly into
the ectoderm, and oralward the outer layer of the bud tends to pene-
trate more and more between the ectoderm and the inner layer.
Therefore I conclude that the inner layer of the bud is constantly
augmented by cell proliferation in its mass, and especially at the neck
of the polypide, and this explanation also accounts for the active cell
proliferation observed at the neck in Plate III. Figure 22, cev. pyd.

Since the polypide later becomes attached to the body wall by the
comparatively narrow " neck " only (Figs. 7, 9, cev. pyd.), a separation of
the oral part from the body wall has to take place. This process begins
at the oral end. In its earliest stages it is indicated by the sharp sep-
aration of the inner bud-layer from the overlying ectoderm, and the

Digitized by VjOOQ IC


partial peDetration of the mesoderm on each side into the space between
these two layers (Plate IV. Fig. 30, ms'drm.). At a later stage the
mesoderm may be seen as a single cell layer lying between the ectoderm
and the inner layer of the bud midway between the oral and anal ends
(Plate IV. Fig. 32, ms'drm.), and as a double cell layer at the oral end of
the bud (Fig. 34^ ms^drm.). It is from these cells at the oral end of the
bud that the retractor muscles are to arise (Plate III. Figs. 23-25, d.
mu, ret). As the oral end of the kamptoderm and oesophagus to which
their inner ends are attached moves away from the ectoderm, and as the
area of the latter itself increases, the two ends of the cells move farther
and farther apart, and the young muscle cells become drawn out into
spindle-shaped muscle fibres. (Plate III. Fig. 25, d. mu. ret. ; Plate IV.
Fig. 36, mu. ret.) The retractor thus arises unpaired and remains so
at its origin, but nearer its insertion in the ring canal and oesophagus
one can distinguish a division into right and left masses. The adult
muscle fibres consist of two parts at least, the inner contractile portion
and an outer less modified protoplasmic portion, which can be traced
over the whole of the first part, but is most evident around the nucleus,
where it has a granular appearance.

6. Pyramidalis, — At about the stage of Figure 25 (Plate III.) one
finds, on cross sections of the branch which pass through the neck of the
polypide, that the mesoderm of the body wall on each side of the neck
is greatly thickened, and that its closely packed cells, which lie three
or four deep, have become somewhat elongated. Cell division is quite
common in the ectoderm of this region, and by it the area of the circum-
cervical region is increased and the two ends of the muscle fibres are
carried farther apart, one end remaining attached to the neck of the
polypide and the other moving towards the abatrial surface. I have
given reasons above (page 16) for believing that the abatrial ends of the
muscles are not carried towards the abatrial side passively, and solely by
the growth of the body wall, but that the ends move relatively to the
cells of the body wall. A somewhat late stage in the development of
the pyraraidalis is shown in Figure 63 (Plate VI.). Nearly the whole
of the mesoderm of the body wall has here been transformed into
muscle cells. The insertion of the muscles is in the mesoderm of the
neck of the polypide. (Plate VI. Fig. 63 ; Plate V. Fig. 45.)

c. Parietal mtiscle^ first make their app>earance at about the stage
of the terminal individual of Plate II. Figure 14, immediately below
the bud and to the right and left, i. e. so that the muscles, which
usually arise paired, have their long axes parallel to the sagittal plane

Digitized by VjOOQ IC


and perpendicular to the long axis of the branch. They arise from cells
of the mesoderm, most of which in this region are filled with vacuoles,
and often project into the coelom. But in my opinion the muscle cells
do not themselves arise from such vacuolated cells, for at even an earlier
stage (corresponding to Figure 21, Plate III.) one can distinguish thick-
ened patches of elongated cells in the mesoderm which are undoubtedly
the young muscle ceUs ; but they do not show the slightest traces of
being vacuolated, and in fact are sharply distinguished from the adjacent
cells by their uniformly granular appearance and their deeper coloration.

Braem ('90, pp. 124, 125) has already stated that the parietal muscles
arise in pairs, and come to traverse the ocBlom, not remaining in the
body wall. The truth of this statement I can confirm in the case of
the parietal muscles first formed, which lie near the future septum.
Plate y. Fig. 42 shows the origin of the muscle fibres on both sides
of the branch. They have already migrated into the coslom. As Braem
plainly states, the component parts of this pair of muscles, developed
from the mesoderm, migrate towards each other and finally fuse into
one unpaired mass, as we see in Plate III. Figure 26. It is perfectly
evident, in this case at least, that both ends of two muscles originating
far apart migrate in some manner towards each other so that the cor-
responding ends come to lie close together. Such a migration cannot
be accounted for merely by growth of the body wall. The ends of the
muscle fibres must move relatively to the body wall.

When the muscles have reached their permanent positions in a
diameter of the branch, we find their ends attached to the cuticula.
As the muscle fibres stain deeply in hsematoxylin, they can be distinctly
traced through the vacuolated and poorly stained cells of the body wall
(Plate III. Fig. 26). Figure 29 shows a bit of the wall mechanically
separated from the cuticula, the end of the muscle fibre remaining in
place. Fine lines can be distinguished in the contractile, deeply stain-
ing portion of the fibre. The surface by which attachment is effected
appears very slightly^ crenulated on longitudinal sections of the muscle
fibre. I could not distinguish any structural peculiarity on the part
of the cuticula to which the muscle was attached, — nothing to indicate
how attachment is effected.

Freese C88, pp. 15, 22, Fig. 11) has described a similar method of
attachment of the muscles to the cuticula for Membranipora.^

1 My friend, Dr. G. H. Parker, tells me that a similar method of attachment of mus-
cle fibres to the cuticula occurs in Crustacea. According to Tullberg ('82, pp. 27, 44,
46), the adductor muscle fibres are in MoUusks attached to the cells of the ectoderm.
The same condition as in MoUusks seems to exist in Annelids (Eisig, '87, pp. 25, 86)

Digitized by VjOOQ IC


At a later stage smaller bundles of muscles arise successively toward
the neck. These muscles are free from the body wall at their middle
region. They do not usually pass through the coelom in a diameter of
tlie branch, however, but rarely subtend as chords an arc of more than
120°. As Braem supposed, such muscles, although arising later than
the most proximal pair, originate in a similar manner to them (Plate
VI. Fig. 55). The mesoderm is very thin at the region at which they
are first seen, and they are quickly discerned by their larger nuclei
and prominent cell body. At a later stage they have grown much
longer, and become freed from the body wall at their middle part.

As is well known, there are two ftmicvli in Paludicella, called by
Allman respectively anterior (nearer the atrial opening) and posterior.
The origin of the funiculi of Paludicella was observed by Dumortier
et van Beneden as long ago as 1850. They say (p. 54), "La couche
muqueuse une fois form^e s'etend rapidement dans Tinterieur et touche
bientot par son extr^mit^ inferieure les parois opposees de la loge.
Les cellules muqueuses dont le tout est encore compose contractent de
Padherence dans cet endroit, et c'est ce qui donne naissance au muscle
r^tmcteur de Festomac [= funiculi]." Allman ('56, p. 36, Plate XI.
Figs. 7-9) also describes and figures very clearly and correctly this pro-
cess, and Braem ('90, p. 127) has recently confirmed their observations.

It is perhaps unnecessary to redescnbe the more evident part of
this process, the contact of the polypide with the abatrial wall of the
branch. The mesoderm of the bud comes into contact with that of
the body wall, the cells of each of the two layers become attached to the
other, and by the withdrawal of the polypide the attachment persists
at two points forming a long drawn out string of tissue. Figures 36*
and 38 (Plate IV.) are contributions to a knowledge of the finer details
of this process. Apparently the upper funiculus is developed earli^
than the lower, as I ^ave always found it longer at about this stage.
The lower funiculus at present consists of only the two mesodermal
layers of body wall and polypide intimately united. The funiculus
itself consists of a cord several cells thick ; but I believe these cer-
tainly to be derived from the mesoderm only. Very early some of
these cells show an appearance of highly refractive and deeply staining
fibres, which I interpret as muscular differentiation (Plate IV. Fig. 38,
fun, «t*.), so that the funiculi must be regarded as partly muscular
in function. As in Phylactolflemata, these fibres lie near the axis of the
funiculus. Braem ('90, pp. ^^^ 67,) has demonstrated that the muscular
fibres of the funiculus of Plumatella pass directly into the muscularis

Digitized by VjOOQ IC


of the body walL It is interesting to find them persisting in the fu-
niculus of Paludicella, beneath the mesodermal covering, although there
is apparently no muscularis developed in the body wall of this region.

8. The Formation of the Neck and Atrial Opening.

This is the last act in the history of the polypide that I shall con-
sider. The body wall around the neck of the polypide continues to
possess a less differentiated character than the remaining portion for
some time after the oral tentacles have undergone their revolution.
One still sees the cells of this region dividing, and the body wall is
gradually protruded at this point above the general level (Plate II.
Fig. 14, cev.pyd.) The neck of the polypide to which the kamptoderm
is attached consists, at a somewhat earlier stage than that just referred
to, of a disk of greatly elongated columnar cells in the centre of which
there is a distinct notch caused by the presence of shorter cells at that
point. (Plate VL Fig. 63 6.) At the inner ends of the columnar cells
of the neck lies a flat epithelium quite sharply marked off from the
latter, but which is nevertheless undoubtedly derived from the same
source as the columnar cells and the inner layer of the bud. This flat
layer is directly continuous with the inner layer of the kamptoderm.
At a later stage, the columnar cells of the ectoderm become elongated
still more, and lose their staining capabilities at their outer ends. Still
later one sees them arranged in the form of a cup whose cavity is sep-
arated from the outside world only by a cuticula which becomes slightly
invaginated at this point. The cells are soon found with their long
axes perpendicular to the edge of the cavity they line.

There is one point that I have not been able to determine ; namely,
how the new cuticula, which is certainly formed at the ends of the cells
which lie next to the cavity, becomes continuous with the old cuticula
of the non-invaginated body wall, as it is in Figure 50 (Plate V.). The
presence on the new nnstainable cuticula of the remains of the stainable
one, whose origin I have already discussed at length, may serve as a
guide to the limits of the old cuticula. The new cuticula is being secreted
by cells lying deep in the inner end of the neck, and apparently in one
rod-like mass. Unfortunately, I lack stages between this figure and
Figure 45 (Plate V.), which shows the neck of a nearly or quite adult
polypide cut lengthwise. The solid cuticular rod has now become a hol-
low cylinder, whose inner (deep) edge is embedded in the deep-lying cells
of the neck. Moreover, one finds superficial to the cuticula of the gen-
eral body wall a second cuticular cylinder, which is free at its outer end,

Digitized by VjOOQ IC


but at its inner end fuses with the surrounding cylinder of cuticula. This
inner cylinder, which is probably formed, as Kraepelin ('87, p. 40) sug-
gested, by splitting of the delicate cuticula at the base of the marginal
thickening (Randwulst), has been compared by Kraepelin to the " collare
setosum " of Ctenostomes. The RandumUt itself I believe to be the equiv-
alent of the Diaphragma of Nitsche, as I shall try to show later.

At the deep end of the neck (Fig. 45), the inner layer of the bud is
seen to be continuous with the ectoderm, llie region of transition may
be called the atrial opening, of, air. Surrounding the atrial opening
is a fold in the ectoderm, and between the layers of this fold is a thin,
non-stainable homogeneous layer, slightly more refractive than the sur-
rounding protoplasm. This membrane extends also a short way into
the kamptoderm, and here lies between its two cell layers. Embedded
m this homogeneous membrane in the fold, one can distinguish still
more highly refractive bodies, tpkt. On account of their form and
high refractivity, I believe these to be muscle fibres cut across. The
homogeneous membrane has also the same general i^pearance and
relation to the muscularis as the so-called supporting membrane of
Nitsche, and it is the only representative of that structure that I
have found in Paludicella.

9. Development op the Communication Plate.

In their description of Paludicella, Dumortier et van Beneden ('50,
p. 40) say : " II se compose de plusieurs loges ou cellules plac^es bout k
bout ... en sorte qu'il n'y a aucune communication entre les diff^rents
animaux." Also Allman (*56, pp. 114, 115) refers to the presence of a
perfectly formed septum separating the cavities of adjacent " cells." To
Kraepelin (*87, p. 38) belongs the credit of having first carefully studied
this structure in the adult by means of sections He came to the con-
clusion from the appearances which he figures (cf. my Plate V. Fig. 49),
that there are small canals passing through the nearly homogeneous
central mass, and therefore " dass wir in dem ganzen Apparat eine Vor-
richtung zu erblicken baben, durch welche Nahrstofflosungen des einen
Tieres -'mittels siebartig wirkender Cautelen in die Korperhohle des
Nachbarindividuums iibergefiihrt werden."

The descriptions of Kraepelin concerning the structure of the '' Roset-
tenplate " are confirmed by my own observations, and se^m to justify his
conclusions cofioeming its function. The development of the organ has
not, however, been carefully observed heretofore. Korotneff ('74, Plate
XII. Figs. 1 and 2) gives figures to show this process, but I have never

Digitized by VjOOQ IC


seen any such circular groove surroundiug the bitmch as he figures. In
all cases the two layers of the body wall form a cu*cular fold, in which,
however, there is never, even at the earliest stages, a space between the
ectodermal layers, nor any infolding of the cuticula as Korotneff (75, p.
369), according to Hoyer's rather incomplete abstract, maintains (Plate
y. Fig. 47). When the circular fold has advanced until only a small pore
remains, by which the cavities of the older and younger individuals are
kept in communication, the mesodermal cells at the angle of the fold
begin to undergo a metamorphosis both in form and histological charac-
ter. In the first place they become much elongated and extremely
attenuated, passing from one surface of the septum to the other, and
forming the lips of the pore. In the second place their plasma becomes
first deeply stainable, and later, in addition, homogeneous and highly
refractive. These metamorphosed cells form what may be called the
ieeUi of the plate. They are derived wholly from mesoderm.

The cells in the upper mesodermal layer next increase rapidly in
number and size, and the number of teeth is also augmented (Plate V.
Fig. 48). The metamorphosis of the cells extends still farther away
firom the communication pore, and involves the lower mesodermal layer ;
but, apparently, each cell of the latter is metamorphosed only to a
Blight depth within its cell wall (Fig. 51), whereas in each of the upper
cells the ends which project into the communication pore are modified
through and through (Fig. 46). At a later stage (Fig. 49) the meta-
morphosed part of the cell seems quite sharply cut off from the active
part, and the slits between the metamorphosed teeth are considerably
reduced. Nevertheless, I believe a transfer of fluids may still occur
between them, for even in the adult communication plate one can trace
continuous lumina when the cells are by accident torn off from the
"teeth" which they have produced. It is important to note that the nu-
clei are not destroyed in the cell metamorphosis. Some lie above, others
below the pore, and become deeply stainable. The ectodermal layers of
the communication plate secrete a cuticula l)etween them. This is thin-
ner than that of the body wall, and does not extend, of course, to the
centre of the communication plate, but ends in a thickened ring, whose
diameter is about one tenth the diameter of the plate, or, absolutely,
about 9.4 1^}

1 Heichert (70, p. 267) first carefully described the Hosettenplate of Cteno-
stomes in Zoiibotryon, and the organ in Palndicella must be regarded as homologous
with it. The central circular hole in the cuticula of Znobotryon is from 7 to 10 /;*
in diameter, and from one ninth to one seventh that of the entire plate. Similar

VOL. XXII — NO. 1. 3

Digitized by VjOOQ IC


10. RdLE OF THE Mesodermal Vacuolated Celi^

AUman ('56, p. 36) observed that at the time a lateral branch was
well formed, and before the origm of the polypide, the mtemal outline of
the body wall was uneven, and he figures (Plate XL Fig. 4) very large
cells lying on the inside of the body wall. Korotneflf (74, Taf. XII.
Figs. 1-3, '75, pp. 369, 370) progressed a step farther, and recognized
a distinction between large, coarsely granular cells projecting into the
cavity of the bud, especially near the tip, and the surrounding epithelial
cells. Braem ('90, p. 126), finally, has described them more accurately.
He finds cells filled with numerous granules in the youngest branches of
the colony. Immediately around the bud, such cells are less abundant ;
probably, he says, because their granules have been absorbed in the
process of formation of the polypide. He compares the granules with
the yolk spherules of the statoblast cells, and believes that they are to
be regarded as food matter.

My observations and conclusions, achieved independently of Braem's,
fully confirm his. I have succeeded, moreover, in obtaining some addi-
tional evidence as to the function of these cells, a subject to which I
have paid some attention.

First as to the distribution of the cells, and their frequency in different
regions. We can best get an approximate idea of this by counting the
number of the reticulated cells in each section of a series which in-
volves a young polypide and the regions immediately above and below
it. It is not possible to do this with perfect accuracy, because there is
no sharp line of distinction between reticulated and non-reticulated cells ;
but I have made the count without prejudice, and I believe as fairly as
possible. When the bud of the polypide has reached about the stage
shown in Plate III. Figure 28, the number of reticulated cells seems to
have nearly reached a maximum. In the series from which this figure
was taken there was an average of 4.8 reticulated cells to the section in
the ten sections distal of the bud. There was an average of 11.2 reticu-
lated cells to the section for the twenty sections which passed through the
bud, and 11.2 for the eleven sections proximal of the bud in the region

perforated organs have been described by Smitt ('67, p. 426), Nitsche (71, pp.
420-422), and Vigelius ('84, p. 26) for Flustra, by Freese ('88, p. 7, 18, 14) for
Membranipora, by OstroumofiE ('86*, p. 18) fop Lepralia, by ClaperMe (70, p. 160)
for Bugula and Scrupocellaria, by Ehlers (76, p. 14) for Hypophorella, and by
Joliet (77, p. 222) for Bowerbankia. Nitsche alone (71, p. 465) has had anything
to say upon their origin, and this apparently not the result of direct observation.

Digitized by VjOOQ IC


at which muscle fibres were arising. A similar series throagh a slightly
older bud gives for the same regions respectively 5, 14, and 13 cells per
section. In series through older buds, a rapid decline in the number of
these cells occurs so that at the stage of Figure 30 (Plate IV.) there is an
average of only abi)ut 3.1 cells per section through the bud, and about
2.2 immediately below. These reticulated cells are not very numerous
in the region of the bud at the time this is about to arise, as a look at
the sections Figures 3 and 4 shows. One finds reticulated cells in the
mesoderm at the tip, and most abundantly at a rather early stage in
the development of the bud. The number of these cells diminishes as
one leaves the young individual to pass into the next older of the same
branch. In the adult such cells are rather rare ; so rare, in fact, that
Kraepelin ('87), who studied with care the body wall of the adult indi-
vidual, makes no mention of them. Nevertheless they do occur in the
cells which are to go into the lateral branch (Plate II. Fig. 15), as well
as elsewhere on the body wall. The place in which one finds the reticu-
lated cells most abundant, however, is in the young lateral branches near
the time when the polypide bud is about to arise. Here every cell of the
mesoderm is greatly enlarged, and filled with the vacuoles (Plate VI.
Fig. 58). These are very apparent upon a surface view of the branches.
Reticulated cells occur not only in the mesodermio cells of the body
wall, but also in those of the polypide bud, which were, indeed, only
lately a part of the mural mesoderm (Plate III. Fig. 28, Plate VI. Fig.
56). Thus, in general terms, we may say that the reticulated cells of
the mesoderm are chiefly confined to regions in which there are young
buds developing; and since these arise at intervals only, there is a
periodicity in their appearance, — a time of maximum development
followed by one of decline, then one of reproduction of such cells in
the ends of branches culminating in another maximum, and so on.

Turning our attention now more particularly to the structure of these
reticulated cells at the period of their best development, we find (Plate
VI. Figs. 66, 57, 59) that they possess a large nucleus lying at the
deep end of the cell and containing a relatively large nucleolus, and that
this is surrounded by a granular protoplasm with included vacuoles. It
is very common to find the nuclei in various stages of division, and thus
it is frequently seen as a mass of chromatic substance without any nu-
clear membrane or nucleochylema. The vacuoles, which in the more reg-
ular cells lie in a semicircle nearly peripheral (the nucleus being at the
centre), are highly variable in number, some of the cells containing as
many as 20 to 30. They often appear as perfectly clear homogeneous

Digitized by VjOOQ IC


spaces, but more frequeDtly at this stage contain a spherical body, which
frequeutly fills the entire vacuole and is more refractive than the sur-
rounding plasma (Fig. 59), Not unfrequently one sees a less refractive,
clear space, surrounding the highly refractive body (Fig. 57).

The description just given corresponds to the condition seen in a ter-
minal branch whose polypide has attained the development of that shown
in Figure 28 (Plate III.). At the time immediately preceding the ori-
gin of the bud, the cuboidal cells of the mesoderm show traces of vac-
uolation, but their form and size have suffered no appreciable disturbance.
This vacuolation of cells proceeds hand in hand with the development of
the bud, and one first notices the homogeneous, highly refractive bodies
in the vacuoles when the bud is well established. At about the time the
alimentary tract has become formed, the reticulated cells begin to show
signs of degeneration. The highly refractive bodies have disappeared,

Online LibraryHarvard University. Museum of Comparative Zoology.Contributions from the Zoölogical Laboratory of the Museum of ..., Issues 21-30 → online text (page 42 of 57)