Jones Quain.

Quain's Elements of anatomy online

. (page 6 of 31)
Online LibraryJones QuainQuain's Elements of anatomy → online text (page 6 of 31)
Font size
QR-code for this ebook


IX WHICH THE PRIMITIVE STREAK HAS BEGUN TO FORM. (Heape.)

The blastoderm is perforated in front of the (short) primitive streak (? blastopore, blp) ; a few
mesoderm-cells are seen anterior to the perforation ; ec, ectoderm ; en, entoderm ; p.s., primitive
streak.

Starting from these premises, and accepting the assumption that the streak represents
the nrastrula-mouth drawn out, the axial increase from before backwards has been interpreted
as signifying the fusion of the lips of the blastopore postulated by the concrescence theories of
His. Minot, and Oscar Hertwig. According to this conception, the bilateral symmetry of the
vertebrate has been brought about by the elongation of the radial gastrula into a cylinder, and
the fusion along the dorsal aspect of the lips of the gastrula-mouth. The fusion takes place from
before backwards, and is manifested by the apparent backward growth of the dorsal lip of the
blastopore, as the embryonal axis is laid down in front of it. This process results in the closing-in




cct




FIG. 71. SECTIONS ACROSS AN AMPHIOXUS EMBRYO. (Hatschek.)

.f/., neural groove ; n.c., neural canal ; ch, rudiment of notochord ; mes. sow., mesodermic somite. In
I. its cavity is in free communication with the alimentary cavity ; ect, ectoderm ; ent, entoderm ;
at, alimentary cavity. In III. the cavity of the somite has extended on either side of the alimentary
canal and forms a ccelom, or body-cavity (cte).

of the archenteron, the roof of which forms the dorsal aspect or axis of the embryo, with the
notochord and segmented mesoderm.

A considerable tide of opinion has in recent years set in in favour of a somewhat modified
conception of gastrulation. Keibel and Hubrecht ' in 1888 independently worked out the con -
ception of gastrulation in two phases; Hubrecht in 1902 named these two stages kephalogenesis
and notogenesis. The primary gastrula, formed in all Craniota by delamination, has a radial
symmetry and forms the fore-part of the head. The second stage of ontogeny, embracing the



See Hubrecht and Keibel, Quart. Jour. Micro. Sc. xlix. 1905.



48 FORMATION OF NEURAL CANAL

formation of the primitive streak and of the notochord, although involving invagination
phenomena, is not to be reckoned as part of the gastrulation process, but represents a
phylogenetic stage by which a radial ccelenterate form was converted into a proto-vertebrate
by the elongation of the gastrula and the formation of a dorsal plate which became the hoto-
chord. The primitive streak is thus not the gastrula-mouth of ontogeny, but represents the
protostoma of an Actinia-like form, as suggested by Sedgwick and Van Beneden.

Assheton's * theory also involves the acceptance of two ontogenetic phases : a first (which
may be exemplified by the earliest phases in Tarsius) resulting in the formation of the forepart
of the head, and a second represented by the formation of trunk and tail. The idea is that the
lips of the circular blastopore grow actively so as to produce a cylindrical gastrula. The dorsal
lip, however, grows more actively in vertebrates, and produces the back and ultimately the tail
or post-anal part of the axis. The anus represents the blastopore, while the mouth is a new
opening (like the gill-slits) into the alimentary canal. Such a process is greatly modified, of course,
in the Amniota, whether those with mesoblastic eggs or mammals, and the ventral lip of the
gastrula is greatly masked by the presence of the yolk-sac. According to this conception, the
primitive streak is only a phase in the development of the embryonic axis out of the growing
blastema of the blastopore-lip, or secondary ' growth- centre.'



EARLY CHANGES IN THE BLASTODERM, RESULTING IN THE
FORMATION OF THE EMBRYO.

FORMATION OF THE NEURAL CANAL, NOTOCHORD, AND
MESODERMIC SEGMENTS.

Neural canal. While the embryonic axis is developing, as described
above, a shallow groove appears on its surface in front of the primitive streak
(fig. 72). This elongates with the axis, and encloses, behind, the anterior end of
the streak with its neurenteric passage. Anteriorly and laterally it is bounded by
a fold of the ectoderm, the groove indeed being produced by the upgrowth of
the limiting folds (figs. 73 and 76). The thickened ectoderm of the groove is called
the neural plate, because the central nervous system is formed from it, and the
bounding folds are termed the neural folds.

By the continued upgrowth of the neural folds (fig. 73) the neural groove is
converted into a deep furrow, and ultimately, by their fusion in the mid-axial line,
into a closed canal (fig. 82, p. 57). The neural plate is then separated from the
surface-ectoderm, and the closed canal becomes isolated as the rudiment of the
cerebrospinal axis. The closure of the canal appears in the human embryo to
begin in the region of the future trunk of the embryo, and proceeds forwards
and backwards. The point where the final closure occurs in front is called the
anterior neuropore. When the neural canal closes posteriorly, the neurenteric canal
comes to lie in its floor, but it is obliterated at an early stage by the fusion of
its lips and soon completely disappears. The anterior end of the neural canal
extends beyond the notochord, and becomes enlarged to form the anterior of
three primary cerebral vesicles round which the brain is formed.

At the point where the lips of the neural folds meet, a mass of ectoderm-cells
forms a thickening known as the neural crest, from which, by a series of changes
afterwards to be described, the nerve-ganglia are formed.

Notochord. It will be recollected that in last section we considered
the development of a plate of cells which we named the notochord-plate. We

1 Anat. Anzeiger, xxvii. 1905.

2 The literature of the germinal layers in mammals and man up to 1902 will be found fully given in
Hertwig, i. Part I. pp. 81 and 949. For a critical review of the earlier literature, see Born in Merkel
and Bonnet's Ergebnisse d. Anat. u. Entwickelungsgesch. i. 1891 ; and of the later literature, Keibel in
the same publication, x. 1901.






NOTOCHOBD



49



saw that, in the early stages, it lies under the neural groove (fig. 64), is directly
continuous on each side with the primitive entoderm, and at the points where it
joins with that layer also with the lateral sheets of mesoderm. By a process of
differentiation from before backwards, pari passu with the axial growth, the plate
now loses its connexion with the mesoderm-plates, although it continues to pass
directly into the lateral entoderm (fig. 76, III.). It next becomes converted
into a rounded rod of cells, at first continuous with, then detached from, an under-
lying layer of entoderm. The mechanism of this process is probably the doubling
up of the notochordal plate and the fusion of the lips of the groove thus formed in
the mid-axial line, just as in the case of the neural canal. The rod of cells thus
formed is the notochord (figs. 81 and 82). The anterior end of the notochord does
not reach to the anterior end of the embryo, but terminates in a recurved
point against the wall of the hypophysis cerebri (epithelial part of the pituitary
body) in the situation of the future body of the sphenoid bone, and close to the
dorsal attachment of the bucco-pharyngeal membrane '(see Development of the
Mouth). It will be seen, there-
fore, that a portion of the neural
canal is prechordal.



It would seem from the data given
for Tarsius by Hubrecht, and also for
the dog by Bonnet, that the anterior
or head end of the notochord is
formed by differentiation directly out
of the primitive entodermic plate
(fig. 78, p. 54) (protochordal plate,
Hubrecht; Ergcinzungsplatte, Bonnet).

The notochord is essentially an
embryonic structure in mammals,
although it does not completely dis-
appear, for traces of it are to be
found throughout life in the middle of
the intervertebral discs. When fully
developed it is a cylindrical rod com-
posed of clear epithelium-like cells,
enclosed within a special sheath of
homogeneous substance. These cells,
although they may become consider
ably enlarged and vacuolated, undergo
no marked histogenetic change and
take no part in the formation of any
tissue of the adult.



amnion



neural groove



neurenteric canal



primitive streak =



abdominal stall' , -j.




FIG. 72. SURFACE VIEW OF EARLY HUMAN EMBRYO, 2 MM.
IN LENGTH. (After Graf v. Spee, from Kollmann's
Entwickelungsgeschichte.) x 80 diameters.

The amnion is opened, and on the blastoderm are seen
the primitive streak, the dorsal opening of the neurenteric
canal, and the neural groove.



Later history of the
mesoderm : formation of
the mesodermic or primi-
tive segments and of the

coelonic At the time when the neural groove is beginning to appear (figs. 73
and 76) a solid sheet of mesoderm extends outwards from the notochordal
plate between ectoderm and entoderm, to be continuous outside the embryonic
shield with the two layers of the extra-embryonic mesoderm. As the neural folds
rise, the central portions of these sheets expand to occupy the spaces, triangular
in section (fig. 73), which the folds enclose. These longitudinal thickenings
gradually thin off laterally into what is known as the lateral mesoderm (fig. 73).
They give origin to the voluntary muscular tissue of the body, and form what
may be termed the paraxial, as distinguished from the lateral mesoderm. These
paraxial thickenings now become cut up by the occurrence at regular intervals,

VOL. i. E



50



MESODERMIC SEGMENTS



transversely across the mass, of a process of thinning into a linear series of
small cubical masses (fig. 74), the mesodermic or primitive segments. 1 The
first pair of these segments appears a short distance in front of Hensen's
knot (fig. 75), in what will ultimately become the junction of the head and
trunk of the embryo. They are produced in succession from before backwards,
being gradually added as the embryonal axis increases in length, until the full
number (thirty-five or more for the human embryo) is laid down. It has been
shown in lower forms that the earliest segment to appear is not the most anterior




IV



-

" '.-.-*




FIG. 73. A SEBIES or TBANSVEBSE SECTIONS THBOUGH AN EMBBYO OF THE DOG. (After Bonnet.)
Section I. is the most anterior. In V. the neural plate is spread out nearly flat.

The series shows the uprising of the neural folds to form the neural canal.
ect, ectoderm; ent, entoderm; mes, mesoderm; so, segment ; c, intermediate cell-mass ; l.p., lateral
plate still undivided in I., II., and III. : in IV. and V. split into somatopleuric (sm) and splanchno-
pleuric (sp) lamellae ; p, pericardium ; h, h, rudiments of endothelial heart-tubes. In III., IV.. and V. the
scattered cells represented between the entoderm and splanchnic layer of mesoderm are the vaso-
formative cells which give origin in front, according to Bonnet, to the heart-tubes (h) ; (a) aortae.

of the series, as a number of head-segments develop from behind forwards in front
of that first differentiating. In the human embryo there are probably three such.
The most anterior segment, in higher vertebrates, lies some distance behind the
head end of the notochord in the future occipital region, and there is no trace of
segmentation in front of this point. 2 As the segments are being cut out of the

1 Formerly known as ' protovertebrae.' The term 'somite ' is also frequently employed to designate
them.

2 In Petromyzon and Selachians the mesoderm is segmented at least as far forwards as the
notochord extends ; the segments in front of the occipital region undergo retrogressive changes, and
disappear at an early stage.



FORMATION OF INTKA-EMBBYONIC CCELOM



51



paraxial mesoderm, each remains attached to the undivided lateral plate by
a continuous tract called the intermediate mesoderm or intermediate cell-mass
(fig. 73, III. c). According to Felix, this continuous tract is formed by the fusion,
At a very early stage, of the stalks of the segments. As the excretory ducts are
afterwards laid down in this tissue, it corresponds to those portions of the hollow
primitive segments which are named the nephrotomes in the Anamnia.

A cleavage has meanwhile taken place in the lateral mesoderm, dividing it into
a parietal and a visceral plate. The parietal plate is associated with the ectoderm
to form the somatopleure, and the visceral plate with the entoderm to form the




I





FIG. 74. PHOTOGBAPH OP A CHICKEN EMBRYO, x 20 diameters. (T. H. Bryce.)

The mesodermic segments, eleven in number at this stage, are seen as small cubical masses on each
side of the axis of the embryo. The eleventh is still continuous with the unsegmented axial mesoderm,
which in turn passes behind into the primitive-streak mesoderm. The neural folds have not united,
and they embrace posteriorly the head of the primitive streak. The optic vesicles are prominent
lateral projections from the fore-brain ; the mid-brain vesicle is visible behind the fore-brain, but that of
the hind-brain is hidden by the tubular heart, which receives posteriorly the two vitelline veins from
the vascular area.

splanchnopleure. The space between the layers becomes the intra-em'bryonic
ccelom (body-cavity), and it follows that when the cleavage reaches the borders of
the shield the intra-embryonic will become continuous with the extra-embryonic
coelom, and the relations of the layers will be established which are reached at a
much earlier stage in lower mammals (fig. 77 ; cf. fig. 49, p. 33).

The segments now also show a small cavity in their interior, round which the
cells arrange themselves in an epithelial fashion. The cavity represents a part of
the coelomic cleft, which in lower vertebrates is continuous with the general
ccelom.

E 2



52 SEPARATION OF EMBRYO

The cleavage first makes its appearance at the anterior end of the axis in the
region where the heart-tubes will be formed. Thence it extends backwards, and
at the same time forwards round the head end of the axis, so that the lateral
coelomic spaces are continuous with one another in front, by a pericephalic cleft
which afterwards becomes the pericardium.

The relations of the layers immediately in front of and behind the axis must
finally be referred to. In the axial line the notochord passes in front into the head-
plate. If this be followed forwards (fig. 78), it will be seen that it is continued into
a portion of the blastoderm between the head end of the axis and the pericephalic
ccelom, into which the mesoderm has not extended (or from which it has
disappeared). The ectoderm and entoderm are therefore here in contact, and form
a membrane known as the buccopharyngeal membrane, which later becomes
perforated to form the mouth-opening. The region of the blastoderm between the
buccopharyngeal membrane and the edge of the shield corresponds to the ' pro-



volk-xic



^ amnion




neurenteric canal
_1 allantoic diveriicnlum



FlG. 75. SUBFACE VIEW OF A BLASTODEEM OF CEBCOPITHECUS CYNOMOLGUS. (After Seleilka.)

The amnion has been opened. The first three segments are visible in front of the neurenteric
canal on each side of the neural groove, which is still open.

amnion ' of lower mammals ; but in the human embryo the ectoderm and entoderm
are, from the first, here separated by mesoderm. This is not split, however, so that
the pericephalic is separated from the extra-embryonic coelom by a bridge of tissue.
Again, at the posterior end of the axis, behind the growing point or tail-knob, the
primitive streak becomes detached from the lateral mesodermic sheets and resolved
into an ectodermal and an entodermal lamella, which together form the cloacal
membrane. This is afterwards perforated to form the urogenital and anal apertures.
Separation of the embryo : history of the yolk-sac and allantois.
As the embryo increases in length, there is a certain increment also in the breadth
of the embryonic shield ; and although the yolk-sac has much increased in size,
the embryo soon begins to expand in all directions beyond the limits of the mouth
of the sac. A folding-in round the margin of the shield, along the line where
amniotic and embryonic ectoderm meet, consequently takes place. The anterior



SEPARATION OF EMBRYO



53



fold first appears (fig. 79), and as a result of the forward growth of the front end
of the axis a diverticulum of the yolk-sac is formed. This becomes in part the
pharynx, but the fore-gut, as the diverticulum is called, is gradually lengthened by
the deepening of the fold and the coming together of the splanchnopleuric folds,
which are nipped in from each side (fig. 76, I.). In consequence of the formation
of the anterior fold, the buccopharyngeal membrane becomes bent in under the



n.f. n.gr



//Kv. ^ *%,\




III.




FIG. 77. TRANSVERSE SECTION THROUGH
A HUMAN EMBRYO OF 2'4 MM.
(T. H. Bryce.)

am, amnion ; n.gr., neural groove ;
not, notochord ; f.g., fore-gut ; y.s., yolk-
sac ; a, aorta of right side ; a.v., allantoic
vein of left side : c, ccelom.

Vessels are seen covering the whole
surface of the yolk-sac.



not.pl



FIG. 76. TRANSVERSE SECTIONS OP THE HUMAN EMBRYO OF 2 MM. REPRESENTED IN FIG. 72.

(After Graf v. Spee.)
In I., which is most anterior, the fore-gut is separated off from the yolk-sac.

n.gr., neural groove ; n.f., neural folds ; ti.pl. (in III.), neural plate ; mes 1 , intra-embryonic mesoderm
still undivided : the commencing intra-embryonic coelom shows as a space (p) in I. to the left, and in
II. on both sides ; it becomes the pericardium; am.ect., amniotic ectoderm ; mes-, amniotic mesoderm ;
ent, entoderm of yolk-sac ; mes 3 , mesoderm of yolk-sac ; not.pl. (in III.), notochordal plate.

head of the embryo and, reversed in position, now forms the still closed anterior
end of the fore-gut. Further, the pericephalic portion of the ccelom, also
reversed in position, comes to lie below the fore-gut, while the bridge of meso-
derm separating it from the extra- embryonic ccplom, and originally at the edge of
the shield, now forms the anterior lip of the primitive umbilical opening, and



54



SEPARATION OF EMBRYO



constitutes what is known as the septum transversum. The folding-in at the
tail end of the embryo takes place rather later, and is complicated by the

presence of the connecting
stalk. In the earliest known
human embryos (fig. 79) there
is a pocket between the
posterior end of the axis and
the upper aspect of the stalk.
As the embryo increases in
length this deepens, the stalk
is displaced forwards, and the

per




primitive streak is bent in to
form, as the anal membrane,
the floor of a diverticulum
named the hind-gut. In front
of the attachment of the stalk
the yolk-sac is further folded
in and the hind-gut is gradu-
ally elongated. Up to this
stage the name connecting
stalk has been applied to the cord of mesoderm uniting the embryonic rudiment
with the chorion. When the tail-fold is produced, it is bent round to the ventral



F IG . 78. MESIAL LONGITUDINAL SECTION THROUGH THE HEAD

END OF THE GERMINAL DISC OF THE DOG BEFORE THE

FORMATION OF THE HEAD-FOLD. (After Bonnet.)

ect, ectoderm of shield ; not.pl., notochordal plate ; p.p.,
primitive entodermal plate (Ergcinzungsplatte, Bonnet) ; Ip.m.,
buccopharyngeal membrane; per, pericephalic portion of
pericardial coelom.

The notochordal plate (archenteric plate, Bonnet) passes
directly into the primitive entodermal plate (Ergcinzungs-
platte, Bonnet).







villas



ammon




- core of villus

^jsg^..^ mesoderm

'connecting stalk
'primitive streak



-yolk-sac



enloderm



mesoderm -



essels



FIG. 79. MEDIAN LONGITUDINAL SECTION OF AN EMBRYO OF 2 MM. (see FIG. 72). (Graf v. Spee.)

aspect of the body of the embryo, and may henceforward be appropriately named
the abdominal stalk (Bauchstiel, His).

Between the hind-gut and the fore-gut there is at first a wide opening into the
yolk-sac (fig. 92), which is gradually reduced to a narrow aperture, and the stalk



ALLANTOIS



55



thus formed is drawn out into a long tubular passage, the vitelline duct, which
widens distally into a rounded vesicle called the umbilical vesicle.

Allantois. In all the Primates the vesicular allantois of lower forms is
represented merely by a narrow tubular passage imbedded in the mesoderm of
the connecting stalk. It appears as a recess of the posterior wall of the yolk-sac
at a very early stage, before the formation of the hind-gut (figs. 53, GO, 61, 79).
This recess is drawn out into a tube as the connecting stalk increases in length.




ect



FIG. 80. DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF THE RABBIT. THE
SECTIONS SHOW THE MANNER IN WHICH THE PRO-AMNION IS FORMED BY A DIPPING DOWN OF

THE HEAD AND ANTERIOR PART OF THE BODY INTO A DEPRESSION OF THE BLASTODERM, WHICH
AT THIS PART IS FORMED OF ECTODERM AND ENTODERM ONLY. THE DIAGRAMS ALSO ILLUSTRATE
THE MODE OF FORMATION OF THE ALLANTOIS AND OF THE TAIL-FOLD OF THE AMNION IN THIS

ANIMAL. (Van Beneden and Julin.)

ect, ectoderm ; ent, entoderm ; me, mesoderm ; cce, parts of the coelom; cce', pericardial ccelom, the
heart not being represented; pr.a., pro-amnion; pi, seat of formation of the placenta; all, allantois ;
am, amnion.



When the stalk is displaced to the ventral aspect, and the umbilical cord is
formed, the passage persists for a time in the cord, while its in tra- embryonic
portion becomes the urachus.

In lower mammals the entodermic diverticulum varies much in the degree of its development.
In the ungulates and carnivores it forms a large vesicle ; in most rodents (fig. 80) it is less
extensive, being confined to the placental site ; in the guinea-pig it is reduced to a tubular
passage in the body-wall and the stalk is a solid cord of mesoderm ; but in all below Primates
the diverticulum, with its covering layer of mesoderm, projects free into the extra-
embryonic ccelom before it comes into contact with the chorion. In the Primates the
embryonic shield is connected from the first with the chorion by the mesodermic
connecting stalk, and the allantois never projects free into the ccelom. The chorion is thus
vascularised directly and not through the agency of the allantois. This close attachment of the



5G



MYOTOMES AND SCLEROTOMES



embryo to the chorion by the short abdominal stalk is accompanied by a certain retardation
of the development of the hind end of the embryo.

Early stages in the development of the muscles and of the
connective tissue and blood-vessels : mesenchyme. It will be recollected
that the mesodermic segments were traced to a stage in which each shows a central
lumen round which the cells are arranged in an epithelial fashion (fig. 82). In
some cases the cavity is occupied by branching cells budded off from the ventral
wall. In transverse section each segment is oval in shape, and now the lower part
of the inner and ventral walls becomes resolved into a mass of loosely arranged
cells, wedge-shaped in section, which encroaches on the cavity (sderotome) (figs. 81
and 82). These cells, along with those in the cavity of the segment, divide actively and
wander inwards, to invest the notochord (fig. 83) and ultimately the neural canal,
in a continuous sheet of loose syncytial tissue known as mesenchyme L (fig. 84).
It constitutes the blastema out of which the axial connective and skeletal tissues
are formed.




FIG. 81. TBANSVEESE SECTION or THE HUMAN EMBRYO OF 2'4 MM. (see FIG. 77), MORE
HIGHLY MAGNIFIED. (T. H. Bryce.)

ent, entoderm of yolk-sac : the lines indicate the points of the splanchnopleuric layers which will
come together to cut off the gut from the cavity of the yolk-sac ; my, outer wall of mesodermic
segment ; me, part of its wall which gives rise to the muscle-plate ; sc, sclerotome ; cce, ccelom. The
other structures as lettered in fig. 77. The amnion, having been torn, is not completed in this
figure.



While the sclerotomes are becoming differentiated, the cavity of the segment
is reduced to a narrow slit bounded by an outer and an inner lamella derived
respectively from the outer and inner wall of the segment (fig. 84). The cells of
the inner lamella elongate, become arranged longitudinally, and are ultimately
(third week) converted into muscle-cells. Hence this lamella is named the
muscle-plate (myotome). It is the rudiment of the voluntary musculature of the
body. The outer lamella of the segment retains its epithelial arrangement for a
time; according to Maurer, it becomes entirely resolved later into a layer of
subcutaneous mesenchyme.



Online LibraryJones QuainQuain's Elements of anatomy → online text (page 6 of 31)