Book - Text-Book of the Embryology of Man and Mammals 6
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Hertwig O. Text-book of the embryology of man and mammals. (1892) Translated 1901 by Mark EL. from 3rd German Edition. S. Sonnenschein, London.
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The Development of the Two Middle Germ-Layers (Coelom-Theory)
AFTER the completion of the gastrula stage the processes of development become more and more complicated, so that the attention of the observer from this time on must be directed to a series of changes which take place at the same time and in various parts of the embryo. For a transformation now ensues, due to the simultaneous folding of both the inner and outer germ-layers, whereby four new chief organs of the vertebrate body are called into existence. Out of the inner primary germ-layer arise (1) the two middle germ-layers, which enclose between them the body-cavity ; (2) the secondary entoclerm or entoblast (Darmdriisenblatt), which lines the secondary intestine of vertebrated animals ; and (3) the fundament of the axial skeleton, the chorda clorsalis, or notochord. At the same time there is developed from the outer germ-layer, as its only system of organs, the fundament of the central nervous system. Since these four processes in the development are in part most intimately involved in one another, they cannot be separated in their treatment.
Here again we have to do with a problem which is one of the most difficult in the embryology of vertebrated animals the history of the development of the two middle germ-layers. Notwithstanding a voluminous literature which has grown out of this theme, there are many conditions, especially among the higher classes of Vertebrata, which are not yet explained in an entirely satisfactory manner. We shall therefore enter somewhat more minutely into this topic, which, like the question as to the origin of the two primary germ-layers, possesses a fundamental significance for the comprehension of the organisation of Vertebrates.
The presentation of what follows will be essentially facilitated, if we allow ourselves a short digression into the history of the development of the Invertebrata, and take under consideration a case in which the middle germ-layers and the body-cavity are established in a manner similar to that which obtains in the case of Vertebrata, but which is easier to investigate arid to understand. Such an example is presented to us in the development of arrov^worms (Sagitta) or Chcetognatha, concerning which observations have been published by KOWALEVSKY, BUTSCHLI, and the author.
- In figs. 66-89 the individual germ-layers are represented in different depths of shade, so as to make their relations to one another more evident. The middle germ-layer is darkest.
After the process of cleavage there arises a typical blastula, which after some time is converted into a typical gastrula. While the latter elongates, two folds of the inner germ-layer arise at the bottom of the coelenteron, and grow up parallel to each other (fig. 65).
Fig. 65. A stage in the development of Sagitta, after KOWAI.F.VSKY, from BALFOUR'S " Comparative Embryology." Optical longitudinal section through a gastmla at the beginning of the formation of the body-cavity. m, Mouth ; ul, alimentary cavity ; pr, body-cavity ; bl.p, blastopore.
Fig. 66. Optical cross section through a larva of Sagittai The coelenteron is separated by means of two folds, which protrude from its ventral wall (T), into the intestinal canal proper and the two lateral body-cavities (111), all of which are still in communication with one another on the dorsal side (Z>). D, Dorsal side ; V, ventral side ; ak, outer, ik, inner germ-layer ; mk\ parietal, mk", visceral middle layer ; //;, body-cavity.
They grow larger and larger, and at the same time stretch over on to the ventral wall of the larva. From here the free edges finally grow on the one hand up to ithe dorsal wall, on the other up to the blastopore, and thereby completely divide the coelenteron into a middle and two lateral spaces (fig. 66 Ih), which for a time communicate with each other near the blastopore and along the subsequent dorsum (D) of the embryo. After a short time this communication is lost ; the blastopore becomes closed, and the edges of the folds fuse with the adjacent surfaces of the ccelenteron. Of the three cavities the middle becomes that of the permanent intestinal tube, the two lateral ones (Ih) become those of the two body-cavity sacs which separate the intestine from the wall of the body. They appropriately take the name enter ocod, since they are formed from the coelenteron by a process of constriction, and are genetically distinguishable from other cavities which arise in other animals between the wall of the intestine and that of the body by simple splitting, and to which is given the ii&m.e jissicoel or schizoccd.
By the process of infolding the number of the germ-layers in Sagitta has been increased from two to three. The primary inner germ-layer is thereby divided into (1) a cell-layer (ik) which lines the intestinal tube, and (2) a cell-layer which serves to enclose the two body-cavities (mk l and mk 2 ). The first is designated as the secondary inner germlayer or entoblast, the second as the middle germ-layer (mesoblast).
One part of the latter is adjacent to the outer germ-layer, the other part to the intestinal tube ; accordingly the division is carried still further into a parietal (mk 1 ) and a visceral layer (mk 2 ) of the mesoblast. For the sake of brevity the former may be called the parietal (mk 1 ), the latter the visceral (mk 2 ) middle layer. Consequently, one may now speak of two middle germ-layers instead of one, the total number of the germ-layers being, naturally, raised by this from three to four.
Fig. 67. Diagrammatic cross section through a young Sagitta.
dM, Dorsal, vM, ventral mesentery ; dh, intestinal cavity ; Ih, body-cavity ; ak, outer, ik, inner germ-layer; wife 1 , "parietal, mk", visceral middle layer (middle germ-layers).
In regard to the course of the further development it may be stated that, while the larva elongates into a worm-like body, the two body-sacs (fig. 67 Ih) are increased to a greater extent than the intestinal tube (ah) which they embrace. They everywhere crowd the latter away from the wall of the body, grow around it from above and below, where their thin walls come into direct contact. By the fusion of the two body-sacs along their surfaces of contact there are formed two delicate membranes, a dorsal (dM) and a ventral (vlf) mesentery, by means of which the intestinal tube is attached to the dorsal wall and to the ventral wall of the trunk.
Processes very similar to those of Sagitta occur in the development of Vertebrata also, but in the latter case they are combined with the development of the neural tube and the chorda dorsalis. In the presentation of these we shall proceed as in the foregoing chapter, which treated of the formation of the gastrula, and consider separately the processes in Amphioxus, Amphibia, Selachians, Birds, and Mammals, since they differ somewhat from one another.
The history of the development of Amphioxus lanceolatus is very instructive. The gastrula elongates, whereby the cceleiiteroii is turned a little towards the future dorsal surface, and here terminates in the blastopore, which marks the future hind end of the worm-shaped body. Then the dorsal surface becomes somewhat flattened ; the cells in this region increase in height, become cylindrical, and form the medullary or neural plate (fig. 69 mp). By a slight infolding of the latter, there arises a medullary groove, which forces downward the roof of the ccelenteron in l '* h Mk the form of a ridge (ck). At the place where the thickened medullary plate joins the small - celled part of the outer germlayer, or the horn-layer (lib), an interruption in the continuity now takes place, and the epidermis grows over the curved neural plate from both sides, until its halves meet in the middle line and fuse. Thus there arises along the back of the embryo (fig. 70) a canal, the lower wall of which is formed by the curved medullary plate (mp), and the upper wall by the overgrowing epidermis (ak). It is only at a later stage that the medullary plate in Amphioxus, lying under the epidermis, is converted into a neural tube (fig. 72 n) by the bending up of its edges and their fusion. As the fundament of the nervous system becomes differentiated, it extends so far toward the posterior end of the embryo, that the blastopore, which is located there, still falls within its territory, and with the closure of the neural tube is included within the end of the latter. Tn this manner it occurs that neural tube and intestinal tube, as KOWALEVSKY first observed, are now, by means of the blastopore, in continuity (fig. 68 en) at the posterior end of the body. The two together constitute a canal composed of two arms, the form of which is comparable with a siphon. The upper arm, which is the neural tube, continues, for a time, to open to the outside world at its anterior end. The bent portion of the siphon, or the blastoporic region, by means of which the neural and the intestinal tube are united, is called canalis neurentericus (fig. 68 en), a structure which we shall again encounter in the development of the remaining Vertebrata.
Fig. 68. Optical longitudinal [sagittal] section through an embryo of Amphioxus with five primitive segments, after HATSCHEK.
V, Anterior, H, posterior end ; ik, inner, nik, middle germ-layev ; dh, intestinal cavity ; n t neural tube ; en, neurenteric canal ; us 1 , first primitive segment ; */*, cavity of primitive segment.
Fig. 69. Cross section of an Amphioxus embryo, in which the first primitive segment is being formed, after HATSCHEK. ale, Outer, ik, inner, ink, middle germ-layer ; Jib, epidermis ; mp, medullary plate ; ch, chorda ; *, evagination of the ccelenteron.
Simultaneously with the neural tube are developed the two middle germ-layers and the chorda dorsalis (figs. 69 and 70). At the front end of the embryo there arise in the roof of the coelenteron close to each other two small evagiiiations, the body-sacs (mk), which grow dorsally and laterally at either side of the curved medullary groove.
These are slowly enlarged, since the process of e vagination progresses from the anterior toward the posterior end of the larva, and finally reaches the blastopore. The narrow strip of the wall of the coelenteron which is found between them and separating them (its limits marked by two Stars * * in figs. 69 and pig 70 _ Cross section of an Amphioxus embryo, 70), and which lies under in which the fifth primitive segment is in . , ,, P ,1 in process of formation, after HATSCHEK.
the middle of the medullary ak> Outer> ik> inner> mlc> mkldle germ . layer; mft groove, represents the funda- medullary plate ; ch, chorda ; *, evagination / 7 7 .7 / 7 v " of the ccelenteron ; dh, intestinal cavity ; Ih, ment oj the chorda (ch), body-cavity.
The primary inner germ layer therefore has noiv undergone division into four different parts : (1) the fundament of the chorda (ch), (2) and (3) the cells (mk) which' line the two body-sacs (Hi) and represent the 'middle germ-layer, and
(4) the remaining part, which, since it is destined to form the bounding wall of the subsequent intestine (dh), is to be designated as permanent entoderm (Darmclriisenblatt) (ik).
The succeeding processes of development have as their objective point the detachment from one another, by means of constriction and fusion, of the parts which are still in continuity, and the formation of discrete cavities. The processes of constriction begin at the anterior end of the embryo, and progress thence to the blastopore (figs. 70 and 71). At first the body-sacs become deeper (fig. 70 Ih), and then lose their connection with the main cavity (dh) by the close apposition of the cells which surround the entrances to them (fig. 71). By. this process the margin of the secondary entoderm (ik} comes to abut directly on the margin of the chordal fundament (ch). The latter has meanwhile also undergone changes ; the plate-like fundament has become so curved by the elevation of its lateral margins, that there has arisen a deep chordal groove, which is open along its ventral side. Subsequently the lateral walls of the groove come into close contact, and are thereby converted into a solid rod of cells, which temporarily shares in the closure of the roof of the secondary intestine, and appears as a ridge-like thickening of the latter. Then the cellrod (ch) becomes detached (fig. 72) from the wall of the intestine ; the latter now, for the first time, becomes completely closed in the form of a tube. To effect this the margins of the entoderm, indicated in fig. 70 l>y stars ( ; *), gi-ow toward each other under the chorda ;md fuse into a median raphe.
Fig. 71. Cross section through an Amphioxus embryo with five well-developed primitive segments, after HATSCHEK.
ale, Outer, ik, inner, ink, middle germ-layer; nip, medullary plate; ch, chorda; dli, intestinal cavity ; Ih, body-cavity.
Fig. 72. Cross section through the middle of the body of an Amphioxus embryo with eleven primitive segments, after HATSCHEK. ,', Neural tube; us, primitive segment. For the meaning of the other letters see Fig. 71.
The final result of all these processes is shown in the cross section tig. 72 : the original ccelenteron has become divided into three cavities -into the ventral permanent intestine (dh), and into the two bodycavities (/A), which are situated dorso-laterally to it, and which continue to increase in size. Between these there has been interpolated the chorda (ch), upon which the intestine abuts below and the neural tube (n] above. The cells which have been cut off from the crelenteron by constriction and which are more deeply shaded in figs. 69 to 72, and enclose the body-cavities (Ih) constitute the middle germ-layer (mk). The part which lies in contact with the outer germ-layer (fig. 72) is recognisable as the parietal middle layer (mk 1 ) ; the part which is in contact with the neural tube, chorda, and intestine as the visceral middle layer (mk 2 ).
Inasmuch as the process of differentiation just described begins, as has been already stated, at the front end of the embryo and extends slowly step by step toward the hind end, by an examination of a series of sections one may follow the various stages of metamorphosis on a single object.
In the description given I have presented the conditions as though in Arnphioxus there arose two simple body-sacs, one on either side of the intestinal tube. The processes are, however, somewhat more complicated, for in the case of the embryo of fig. 70 the body-sacs, while increasing in size posteriorly, undergo further changes in the anterior region, and through repeated infoldings are divided into separate compartments, the primitive segments (us), which lie one behind the other. I content myself with this statement, since for didactic reasons I shall defer the treatment of the development of the primitive segments until I come to a subsequent chapter.
While in the case of Amphioxus lanceolatus there is no doubt but that the body-cavity and the middle germ-layer are formed by an outpocketing of the watt of the codenteron, opinions upon the origin of the same parts in the case of the remaining Vertebrata are still very divergent. This results, in the first place, from the fact that the investigation, which can be carried out only by means of serial sections, is coupled with greater technical difficulties, and, secondly, because the conditions are somewhat altered, owing to the greater abundance of yolk in the eggs, and furnish less clear and intelligible views. Where in the gastrula of Amphioxus a great cavity is present, we see in the case of the remaining Vertebrates a great mass of yolk-material collected, and the ccelenteron more or less completely filled with it. Consequently there are formed in these cases for the production of the body-cavity no holloiv evaginations, but solid cell-growths, in that the parietal and the visceral lamellce of the middle germ-layer have the surfaces which inAmphioxus bound the bodycavity pressed together at the beginning of the development and separated onli/ at a, rather late stage. In order to make easier the comprehension of the somewhat dissimilar appearances furnished b an investlgation of the separate
73 Diagram to show the development of the middle germ-layers and the body-cavity in Vertebrata. Cross section of an embryo in front of the blastopore.
Medullary plate; ch, fundament of the chorda; ak, classes of Vertebrates, outer, ik, inner gei-m-layer ; mk 1 , parietal, rnk", visceral , -, M 'fU lamella of the middle germ-layer; d, yolk-mass; dk, 5t, Wltn yolk-nuclei ; dh, intestinal cavity ; Ih, body-cavity.
the aid of two diagrammatic figures, how, according to a series of investigations which I have undertaken, the development of the middle germ-layer and the body-cavity would take place in the case of the vertebrated animals.
One of the diagrams (fig. 73) represents a cross section in front of the blastopore. It exhibits the inner germlayer (ik) extensively thickened on the ventral side by the deposition of yolk (d), so that the crelenteron is reFig. 74. Cross section of an Amphioxus embryo. duced to a Small cavity (dh).
In the roof of the co3lenteron there lies a single layer of cells (ch), the fundament of the chorda, characterised by their cylindrical form. On both sides of it the inner germ -layer has developed evaginations, the two body-sacs (Ih), which have grown down some distance between the yolk-mass and the outer germ-layer. Their wall (mk l and mk y ) is composed of small cubical or polygonal elements, shaded darker in the diagram. The ccelenteron is distinctly separated by means of the two ccelenteric folds (* *) into a median or intestinal cavity proper (dli), lying beneath the chordal fundament, and the two narrow body-sacs (Hi), which communicate with the former only by means of narrow fissures (* *) at the right and left of the chordal fundament. The figure is easily reducible to the preceding (p. 113) cross section of an Amphioxus embryo (fig. 74), if we conceive the simple epithelium on the ventral side of the latter thickened by an accumulation of yolk, and the two small body-sacs grown down a certain distance between yolk-mass and outer germ-layer.
See explanation of Fig. 70. ale, Outer, ik; inner, mk, middle germ-layer; ch, chorda.
In the second diagrammatic cross section, which is through the blastopore (fig. 75), the ccelenteroii (ud} is wholly filled up with the yolkmass (d). The body-sacs (Ui) described in the first diagram are to be seen here also, as they crowd themselves downwards between yolk and outer germ-layer. Their walls are composed of small cells, and the outer or parietal layer (mk l ) merges into the outer germ-layer at the blastopore, while the inner or visceral layer (mk 2 } is continuous with the yolk-mass or the inner germ -layer.
Were the conditions in Vertebrates such as the two diagrams represent, there could no longer be any doubt in regard to them, any more than in the case of Amphioxus, that the body-cavity is developed out of two evaginations of the coelenteron, and that its walls constitute the two middle germ-layers. But there is not a single Vertebrate which presents such clear and convincing evidence. The distinctness is everywhere diminished, most of all by the fact that the parts which are to be interpreted as body-sacs no longer enclose cavities, because their walls are firmly pressed together, in consequence of the fact that the greater collection of yolk requires the space for itself. Consequently we find, in place, of the body-sacs exhibited in the diagram, solid masses of cells, for which it remains to be established that then correspond to the sacs in position and development.
Fig. 75. Diagram to show the development of the middle germ-layers and the body-cavity in Vertebrata. Cross section through the blastopore of an embryo.
u, Blastopore ; ud, coelenteron ; Ui, body-cavity ; d, yolk ; ak, oniev germ-layer ; mk\ parietal, mk'-, visceral lamella of the middle germ-layer.
In order to see what condition would result in consequence of a disappearance of the body-cavity, we will imagine that in the two diagrams the parietal and the visceral layers of the body-sacs are firmly pressed together. In the first diagram (fig. 73) we should then have a mass several cells thick, which would be everywhere distinctly separated from the two germ-layers in between which it had grown with the exception of the place indicated by a star, which marks the entrance to the body-sac ; this is the important region whence the evagination or the outgrowth of the middle germ-layer from the inner layer has taken place. At this point the cell-mass is continuous, on the one side with the fundament of the chorda, on the other with the entoderm. In the second diagram (fig. 75) we should likewise see the thick cell-mass everywhere isolated, except in the vicinity of the blastopore, where a transition to the outer as well as to the inner germ-layer takes place. If, in addition to this, we should imagine that the two lips of the blastopore were here pressed together from right to left, we should have in the middle of the cross section a thick, many-layered cell-ma^s, which on both sides is resolved into the three germ-layers, or, in other words, at the blastopore all three germ-layers Ixj their fusion meet together in a singlemass of cells.
By careful investigation it is, in fact, demonstrable that similar conditions to those which we have produced by changes in the diagrams are found in the investigation of the several classes of Vertebrates. For this purpose we must make sections through three different regions of the embryo : (1) through the region in front of the blastopore, (2) through the region of the blastopore itself, and (3) behind it. The agreement appears most prominent in the development of the Amj)hibia, among which the Tritons again furnish the most instructive objects.
When in the case of Triton the gastrulation, with the accompanying obliteration of the cleavage-cavity, is fully completed, the embryo becomes slightly elongated; the future dorsal surface (fig. 76 D] becomes flattened, and gives rise to a shallow furrow (?), which stretches from the anterior to the posterior end nearly up to the blastopore (u). The latter has now assumed the form of a longitudinal fissure. A cross section made through the middle of the embryo in front of the blastopore (fig. 77) corresponds in every particular to our first diagram (fig. 73), if we conceive that the body-cavity in this case has disappeared. The outer gerin-layer (ak) consists of a single sheet of cells, which on the back of the embryo are cylindrical, but become shorter toward its ventral side. The tf cells enclosed within the outer layer exhibit a differentiation in three ways, and therefore are subsequently converted into three different organs into chorda, entoderm, and middle germ-layer. First, there is to be found 011 the roof of the ccelenteroii (dh) under the medullary groove, even close up to the blastopore, a narrow band of long cylindrical cells (ch) ; it corresponds in every respect to the fundament of the chorda in our diagram (fig. 73 ch), and in the cross section through Amphioxus (fig. 74 ch). Secondly, the fundament of the chorda is flanked on either side by two bands (mk 1 , ink 2 ) of small oval cells, which extend downwards to about the middle of the lateral region of the embryo. They do not share in bounding the ccelenteron, since a third kind of cells (ik), large and rich in yolk, lie along their inner surfaces. The latter begin at the margin of the chorda! fundament as a single layer, become two layers thick farther down, and thus merge into the more voluminous accumulation of yolk-cells, which, in all Amphibian embryos, occupy the ventral side and restrict the gastrula-cavity. They correspond, to continue with our comparison, with the entoderm, whereas the small-celled masses, which, starting from the fundament of the chorda, have crowded themselves out between the entoderm and the outer germ-layer, are comparable with the cells which in Amphioxus and in our diagram form the wall of the body-sacs, or the middle germ-layer. The conclusion is therefore justified and very obvious, that in Triton the tivo middle yerm-layers have arisen in the anterior territory of the embryonic body by a process of evagination at both sides of the chordal jfundament, just as in Ampkioxus, except that in one case the evayinated cell-mass contains a cavity, in the other case none.
Fig. 76. Egg of Triton with distinctly developed medullary groove, seen from the blastopore, 53 hours after artificial fertilisation. D, Dorsal, V, ventral region ; u, blastopore ; h, elevation between blastopore and medullary groove (r) ; /, semicircular furrow, which encloses the blastoporal area ; dp, yolk-plug.
Fig. 77. Cross section of an egg of Triton with feebly expressed medullary groove. ak, Outer, ik, inner germ-layer ; mk 1 , parietal, mk", visceral lamella of the middle germ-layer ; ch, chorda; dh, intestinal cavity ; D, dorsal, V, ventral.
Fig. 78. Cross section through the blastopore of an egg of Triton with feebly expressed medullary groove.
ak, Outer, ik, inner germ-layer ; mk l , parietal, mk'*, visceral lamella of the middle germ-layer; u, blastopore ; dz, yolk-cells ; dp, yolk-plug ; dh, intestinal cavity.
A cross section through the blastopore of the Triton embryo (fig. 78) is to be compared with our second diagram (fig. 75). The hollow body-sacs of the latter correspond to the solid cell-bands, which are the fundament of the middle germ-layer. Near the blastopore (u) they are split into two lamellse. Of these the outer (mk l ) merges, as in our diagram, into the inner layer of the blastoporic lip, and becomes continuous at the edge of the blastopore with the outer germ-layer (ak) ; the inner lamella (mk 2 ), on the contrary, is connected with the mass of yolk-cells (dz), which lies like a wall in front of the blastopore and even projects into it as the RUSCONIAN yolk-plug (dp).
Posteriorly to the blastopore, the middle germ-layer stretches itself out for some distance, but here only as a single connected mass.
According to the region from which the middle germ-layer is developed, we may divide it into two portions, and call that part which is produced 011 both sides of the chorda the gastral mesoderm, and that which arises from the blastopore the peristomal mesoderm (RABL).
+++++++++++++++++++++++++++++++++++++++++ Fig. 79. Three cross sections from a series through an egg on which the medullary ridges begin to appear. The sections illustrate the development of the chorda out of the chordal fundament, and the constricting off of the two halves of the middle germ-layer.
ak, Outer, ik, inner germ-layer ; mk l , parietal, mk", visceral lamella of the middle germ-layer ; mp, medullary plate ; mf, medullary folds ; cJt, chorda; Ih, body-cavity. +++++++++++++++++++++++++++++++++++++++++
The further development of the fundaments of mesoderm, chorda, and intestine, which subsequently become entirely separated from one another at the places where they now remain in connection, causes the agreement with the conditions found in Amphioxus to appear in stronger relief. The process of separation is introduced by the curving of the chordal plate, and its conversion into the chordal groove (fig. 79 A ch). Inasmuch as it is continuous at its edges with the parietal lamella of the middle germ-layer (mkty, there arise in the roof of the coeleiiteron the two small chordal folds, which enclose between them the chordal groove. Its free margins abut directly upon the folded edge, where the visceral lamella of the middle germ-layer (ink' 2 ) bends around into the entoderm (ik) to produce the coelenteric fold.
In the next following stage (fig. 79 B] the thickened medullary plate, consisting of long cylindrical cells, becomes distinctly marked off from the now still smaller cubical elements of the ectoderm. Meanwhile the middle germ-layer begins to detach itself from its previous connections in the vicinity of the place of evagination ; the parietal lamella becomes separated from the fundament of the chorda, the visceral lamella from the entoderm, and thereupon their detached edges become fused to each other. By means of this process the fundament of the body-sac, or of the middle germ-layer, becomes closed 011 all sides, and is separated from the other germ-layers. At the same time the entoderm (ik} and the fundament of the chorda (ck) have come into contact along their free margins, so that the chorda appears like a thickening of the entoderm, and for a time shares in bounding the intestinal cavity on the dorsal side. This is changed by a second process of detachment.
The fundament of the chorda, now converted into a solid rod, is gradually excluded from participation in lining the intestine (fig. 79 C), by the fact that the halves of the entoderm (ik), composed of large yolk-cells, grow toward each other underneath it, and fuse in a median raphe.
The closure of the permanent intestine on the dorsal side, the constricting off of the two body-sacs from the inner germ-layer, and the origin of the chorda dorsalis are therefore in Amphibia, as in Amphioxus, processes ivhich are most intimately related with one another. Here, too, constricting off of the parts 'mentioned, begins at the head-end of the embryo, and advances slowly toward the posterior end, where there exists for a long time a zone of growth, by means of ivhich the increase in the length of the body is effected. Soon after this, the moment arrives w r hen in the embryos of Triton the body-cavity becomes visible. For after the detachment of the organs previously mentioned is completed, the two middle germ-layers at the head-end of the body, and on both sides of the chorda, separate from each other, and thus cause to appear a right and a left body-cavity (enterocoel), which, according to my interpretation, were not previously recognisable, simply on account of the intimate mutual contact of their walls.
Meanwhile the medullary plate has become converted, by the process of folding already described, into the neural tube (fig. 80 me), which lies beneath the epidermis. Since the neural tube subsequently encloses the blastopore, and is thereby in communication with the intestinal tube (as the preceding longitudinal section of an advanced embryo of Bombinator most distinctly shows), it follows that there is also in the Amphibia a structure (fig. 80 ne) corresponding to the neurenteric canal of Aniphioxus (compare fig. 68 en).
Fig. 80. Longitudinal [sagittal] section through an advanced embryo of Bombinator, after GOETTE.
m, Mouth ; an, aims ; I, liver ; ne, ueureuteric canal ; me, medullary tube ; ch, chorda ; pn, pineal gland.
More fundamental differences in the development of the middle germ-layer are met with in the eggs of Fishes, Reptiles, and Birds, which are more abundantly provided with nutritive yolk and undergo partial cleavage
+++++++++++++++++++++++++++++++++++++++++ Fig. 81 A and B. Two germ-discs of Hens' eggs in the first hours of incubation, after ROLLER. c7/, Area opaca ; hf, area pellucida ; s, crescent ; sfc, crescent-knob ; Es, embryonic shield ; pr, primitive groove. +++++++++++++++++++++++++++++++++++++++++
in the eggs of Mammals. However, the variations appear in these cases to be of a subsidiary nature, whereas in the chief points the unity of the developmental processes for all vertebrated animals has been the more firmly established the more accurately the individual stages have been investigated by means of improved methods.
In the presentation of these difficult conditions, we shall describe first the changes which may be recognised in viewing the germ-disc from the surface, and to these shall add, secondly, the more important results acquired by series of cross sections.
At the posterior margin of the germ-disc of the Chick (fig. 81 A), which consists of two layers lying on the yolk like a watch-glass, we had distinguished not only a short time before incubation, but also during the early hours of that process the crescent (s) and the crescentic groove, and had learned to recognise that this was the place from which the inner germ-layer arose by a process of folding under.
When, during the first hours of incubation, the germ-layers grow out farther on the yolk, the crescentic groove (fig. 81 E} is converted into the primitive groove (pr), a structure of far-reaching significance.
The metamorphosis, according to the excellent researches of DUVAL, takes place in the following manner : In the middle of the anterior blastoporic lip, where the outer germ-layer bends over to become continuous with the inner, there arises a small notch, which is directed forwards (fig. 81 A sty ; this gradually elongates into a groove (fig. 81 ), corresponding with the future longitudinal axis of the embryo, and by the following method : the right and the left halves of the [anterior] blastoporic lip, together with the part which bounds the first notch, grow toward each other, and come in contact with each other in the median plane, with the same rapidity with which the disc increases in superficial extent. For a time,
therefore, the blastopore has the form of a short longitudinal groove, which, at its posterior end, is beilt around into pi gi 32. Diagrams to elucidate the formation of the primi.
two short transversely tive groove, after DUVAL.
The increasing size of the germ-disc in the course of the placed Crescentic horilS development is indicated by dotted circular lines. The (s\ Finallv these also heavy lines represent the crescentic groove, and the primitive groove which arises from it by the fusion of have disappeared ; they, the edges of the crescent.
too, have grown toward each other, toward the median plane, and have thus contributed largely to the posterior elongation of the primitive groove. By this remarkable process of growth the whole blastopore is converted from a transverse fissure into a longitudinal one.
The accompanying diagrams (fig. 82) serve to illustrate this highly important process. The increase which the germ-disc has undergone during successive stages is indicated by dotted lines. The margin of the fold, where the upper germ-layer passes over into the lower layer, or the anterior lip of the blastopore, is denoted by a heavy black line. In the figures A, B, C, one observes how, with the increasing extent of the germ-disc, the right and left halves of the blastoporic lip coine together in the median plane in ever-increasing extent, and form the primitive groove.
In figs. 83 and 84 are presented instructive cross sections through the primitive groove in the first stages of its development. The first shows us the two lips of the blastopore (fig. 83?^), separated by a small space, into which there projects from below a small elevation (dp) of yolk-substance, containing a number of nuclei (merocytes), comparable with the RUSCONIAN yolk-plug in the Amphibian larva (fig. 78 dp). At the lips, the upper germ-layer, a single cell thick, bends around into the lower germ-layer, composed of loosely associated cells. The blastopore leads into the coelenteroii, which lies between yolk and germ-disc. In fig. 84 the margins of the two folds have come into close contact, and have fused to form the anterior part of the primitive streak, above which the primitive groove is still to be found.
When the last remnant of the crescentic groove has been employed for the elongation of the primitive groove, the margin of the germdisc, which continues all the time to spread itself out uniformly over the yolk, exhibits everywhere one and the same condition ; it has become at all points a circumcrescence-margin, now that the invayination-maryin has detached itself from it as primitive groove.
+++++++++++++++++++++++++++++++++++++++++ Fig. 8(3.
Fig. 85. Surface view of the area pellucida in the blastoderm of a Chick, soon after the formation of the primitive groove, after BALKOUR. pr } Primitive streak with primitive groove ; a/, aniniotic fold. The darker shading surrounding the primitive streak indicates the extent of the niesoblast. +++++++++++++++++++++++++++++++++++++++++ Fig. 86. Surface view of the area pellucida of a blastoderm of 18 hours, after BALFOUR. +++++++++++++++++++++++++++++++++++++++++ The area opaca is omitted ; the pear-shaped outline marks the limit of the area pellucida. At the place where the two medullary folds are continuous with each other there is to be seen a short curved line, which represents the head-fold. In front of it there lies a second line concentric with it, the beginning of the amniotic fold. A , Medullary folds ; me, medullary furrow ; pr, primitive groove.
When subsequently the pellucid and opaque areas become more distinctly separated, the primitive groove comes to lie in the posterior part of the pellucid area. By careful examination of a surface preparation (tigs. 85 and 86 pr), one sees that it is bounded, both on the right side and on. the left, by two small folds, which are derived from the blastoporic lips, and which appear darker and more opaque because the cells are multiplying rapidly and are more closely crowded. Since the two primitive folds, or the two blastoporic lips, are closely in contact at the bottom of the groove, and indeed are in places completely fused, they together produce in the pellucid area a dark streak of substance, which is about a millimetre long and 0'2 mm. broad. With the earlier embryologists, to whom it was already known, we designate this as the primitive streak of the germ-disc. In the vicinity of the primitive streak there are to be distinguished in surface views, now and during the following stages of development, some additional changes, which are caused by the beginnings of special organs. In the first place, there is to be seen in the anterior region of the area pellucida, and in the direct continuation of the primitive streak, a narrow, dark streak of cells, which has been designated by KOLLIKER as the head-process of the primitive streak, and which gradually increases in length. Secondly, there appears an increasing opacity (fig. 85) in the vicinity of the primitive streak and its head-process, which afterward stretches
+++++++++++++++++++++++++++++++++++++++++ Fig. 87. Blastoderm of the Chick, incubated 33 hours, after DUVAL. +++++++++++++++++++++++++++++++++++++++++ The area pellucida (/</) is surrounded with a portion of the opaque area (('/). The fundament of the nervous system is nearly closed in front and segmented into the three brain-vesicles lib 1 , hit'-, A6 3 ; behind, the medullary furrow (//;/) is still open. On either side of the latter there are six primitive segments (its). The posterior end of the embryonic fundament is occupied by the primitive streak and the primitive groove out farther laterally : it is connected with the origin of the middle germ-layer.
In a still later stage of development (fig. 86), at the beginning of the second day of incubation, the first fundament of the central nervous system makes its appearance in the anterior portion of the germ-disc. Over the head-process there arise at some distance from each other the two medullary folds (A), which are continuous with each other at their anterior ends, and which bound the broad medullary furrow (me] ; posteriorly they become less prominent, and they here embrace between them the anterior end of the primitive streak (/>?*). Medullary furrow (me} and primitive groove (pr] must not be confounded with each other, as occurred in the earlier days of embryology ; they are two entirely distinct and dissimilar structures, which exist at the same time, and independently of each other, as fig. 86 shows.
Primitive streak and primitive groove are preserved for a longtime without undergoing important changes (fig. 87 pr). They always occupy the posterior end of the embryonic body, which is characterised by its slightly differentiated condition even in stages when the development of the separate organs of the body is already in full progress. On the contrary, the embryonic territory lying in front of it, which is so small at the time of the appearance of the head-process, becomes greatly elongated and, at the same time. differentiated into the separate organs of the body. This process of differentiation begins in front, and proceeds posteriorly toward the primitive groove, just as in Amphioxns and the Amphibia. The margins of the medullary folds come into contact with each other and begin to fuse, forming the neural tube (hb l , hb 2 , hb*, mf], the fusion progressing from the head- toward the tail-end. There are also to be recognised now in the interior of the body, at either side of the neural tube, the protovertebrse or primitive segments (us), which we shall investigate more minutely further on. The number of these is constantly increased by the growth which is taking place at the tail-end.
When a large number of primitive segments has arisen, the primitive groove begins on surface-views to disappear ; for it is surrounded by the medullary folds, and inasmuch as these fuse here as well as elsewhere, it is enclosed in the terminal part of the neural tube. A notable condition, and one of great importance for the interpretation of the primitive groove, has been discovered at this stage in the embryos of several species of Birds by GASSER, BRAUN, HOFFMANN, and others. At the front end of the primitive groove a narrow canal has arisen, which leads obliquely from the neural tube under the entoderm, and unites the two in the same manner in which the blastopore does in Amphioxus and the Amphibia. A diagrammatic longitudinal section through the hind end of a Chick (fig. 88) shows us this important union (n.e), which exactly corresponds to the condition of an Amphibian embryo presented in fig. 80.
Such a neurenter i c canal has been observed still more distinctly in Selachians and Reptiles and at even e a r 1 i e r s t a g e s, whereas in Teleosts it does not come to development on account of special subsidiary conditions.* The investigation of the embryonic fundaments of a Mammal fur* nishes us with views quite similar to those respecting the Chick. When In Selachians the blastopore is very earl} 7 enclosed within the medullary folds, and then assumes the condition of a long-persisting canal-like passage to the intestinal cavity through the floor of the medullary groove, and later through that of the neural canal.
In the case of Reptiles, the primitive streak is very short and triangular, and in many species soon discloses, before other organs have been differentiated, an opening at its anterior end which leads to the cavity under the germ-disc, which is filled with yolk. Subsequently the opening is converted into a canal, the wall of which is composed of cylindrical cells, and is in continuity above with the outer germ-layer, and below with the inner germ-layer. Then the medullary folds, which are being formed in front of the orifice, grow around it ; the orifice now becomes a genuine neurenteric canal, which in many cases appears to become obliterated even before the closure of the medullary tube, but in other cases persists for a long time.
+++++++++++++++++++++++++++++++++++++++++ Fig. 88. Diagrammatic longitudinal section through the posterior end of an embryo Chick at the time of the formation of the allantois, after BALFOUR.
The section shows that the neural tube (Sp.c) is continuous at its posterior end with the post-anal intestine (p.a.g) by means of the neurenteric canal (n.e). The latter traverses the remnant of the primitive streak (pr), which is folded over on to the ventral side, ep, Outer germ-layer ; ch, chorda ; hy, entoderm ; al, allantois ; me, middle genii-layer ; an, the place where the anus will arise ; am, amnion ; so, somatopleure ; sp, splanchnopleure. +++++++++++++++++++++++++++++++++++++++++
the embryonic area has assumed an oval form, the opacity at the posterior end, or the terminal ridge (fig. Q3hw), which was compared with the crescent of the Bird, elongates into the primitive streak ; the latter occupies the posterior half of the embryonic area (fig. 89 A pr), and exhibits a distinct groove, that is flanked by aright and a left ridge-like fold. (Compare with this the Chick as shown in fig. 85.)
+++++++++++++++++++++++++++++++++++++++++ Fig. 89 A. Embryonic fundament of an 8-days Rabbit, after KOLOKER. arg, Fundament of the embryo ; pr, primitive streak.
Fig. 89 B. Vascular area (o) and embryonic fundament (#) of a 7-days Rabbit's egg, after KOLLIKER. o, Vascular area (area opaca) ; ar/, embryonic fundament ; pr, primitive groove ; rf, medullary furrow. +++++++++++++++++++++++++++++++++++++++++ Afterwards there appears in this instance, just as with the Chick, a narrow opaque streak in the forward prolongation of the primitive streak, its head-process, and this divides the anterior portion of the germ into a right and a left half (fig. 90 &/"). After some time there are developed on both sides of the head-process the medullary folds (fig. 89-6), which bound the broad medullary furrow (rf}, and which, by forming a bow at their anterior ends, become continuous with each other ; but posteriorly they diverge somewhat from each other, and embrace the primitive groove (pr}. This stage corresponds to the condition of the Chick presented in fig. 86.
From this time forward the anterior part of the embryonic area grows in length much more rapidly than the hind part with its primitive groove ; the latter remains almost unaltered in Mammals up to late stages of development, and then diminishes in length, not only relatively, but also absolutely.
Fig. 90. Germ-disc of an embryo Rabbit with primitive streak, after E. VAN BENEDEN. pi', Primitive streak ; Jcf, head-process ; hlc, HENSEN'S node ; cn, canalis neurentericus.
Fig. 91. An embryo Rabbit with a part of the area pellucida 9 days after fertilisation. Magnified 22 diameters. After K'O'LLIKER.
ap, Area pellucid i; ao, area opaca; h', medullary plate in the region of subsequent first brainvesicle ; h", the same in the region of the subsequent mid-brain, where the medullary furrow (}/) exhibits a widening ; h'", the same in the region of the subsequent third brainvesicle ; hz, fundament of the heart ; stz, trunk zone (Stammzone) ; 212, parietal zone ; pr, remnant of the primitive streak.
At the same time the embryonic area passes from the oval to a pronounced guitar-shaped outline. Such an embryo is represented in fig. 91. The primitive streak (pr} is to be seen at its posterior end, partly embraced by the medullary folds (rf). The middle germlayer is already fully developed, and in the future neck-region three pairs of primitive segments have already been differentiated at the sides of the chorda.
Just as there has been up to this stage an 'agreement with Birds and Reptiles in other points, so there also is in the existence of a neurenteric canal. At a rather early stage there is already noticeable, at the anterior end of the primitive streak, a small spot, at which, in consequence of cell-proliferation, a large amount of material is accumulated. It is known under the name of HENSEN'S node (fig 90 hk}. This is important chiefly because a narrow canal, the canalis neurentericus (en), passes through it, and leads from the outside into the interior of the blastodermic vesicle. The presence of this canal has already been established by several investigators by VAN BENEDEN in the Eabbit and the Bat, by BONNET in the Sheep, by HEAPE in the Mole, and by GRAF SPEE in a young human embryo. The latter exhibited a still widely open medullary furrow. At the beginning of the primitive groove there was a wide, roundish, triangular orifice, which traversed the germ-disc, and was surrounded by a ring-like elevation corresponding in position to HENSEN'S node.
I have dwelt upon the primitive streak more at length, and have considered more in detail its first appearance and its topographic relations to other organs, because from a developmental standpoint it is a very important structure, and one the significance of which is still much discussed. For it corresponds to the blastopore of the lower Vertebrates, and LS important as the region from which the middle germ-layer takes its origin. While I postpone an exposition of the grounds which warrant us in designating the primitive groove as blastopore, I shall at once consider the development of the middle germ-layer. Information concerning this is to be got from cross sections, which should be made, as in the Amphibians, (1) in front of the primitive groove, (2) in the region of the groove, and (3) back of it, both in younger and older embryos.
In embryonic fundaments which have reached the stages represented in figs. 81 B, 85, and 89, the middle germ-layer is already begun in the immediate vicinity of the primitive groove, and causes the opacity which appears upon both sides and in front of it. Cross sections through the cephalic process of the primitive streak now allow the establishment of a complete agreement in one fundamental point between Amphioxus and the Amphibia on the one hand, and Selachians, Reptiles, Birds, and Mammals on the other.
Along a narrow median streak, in the former groups in front of the blastopore, in the latter in front of the primitive groove, the embryonic fundament is composed of only two germ-layers, of which the lower is destined to become the chorda. At both sides of these regions the twolayered condition yxisses abruptly in all Vertebrates into a three-layered one, the outer germ -layer being followed by the middle layer, and this by the inner germ-layer.
Fig. 92 A and B. Cross sections through the germ-disc of a Selachian. Copy after BALFOUR'S Monograph, PI. IV., Fig. 8a, and PI. IX., Fig. la. Only the left half of section A is represented. ak, Outer, ik, inner, mk, middle germ -layer ; ch, chorda ; ;np, medullary plate ; <l , yolk.
The conditions in detail assume in Selachians, Birds, and Mammals the forms indicated by the accompanying figures (92-95).
In the Selachians the medullary fold is well marked in cross sections (fig. 92 A mp). Beneath it there lies, as in Amphioxus and Triton, only a single layer of tall cylindrical cells (ch), the fundament of the chorda ; laterally this merges into a many-layered mass of small cells, which is soon divided by means of a fissure into two distinctly separated lamellae into the middle layer (mk), composed of small polygonal cells, and into the inner layer (ik"), which here consists of a single layer of tall columnar cells. At the point indicated by a star, the fundament of the chorda and the middle and inner germ-layers are continuous with one another. At a later stage (fig. 92 B) a separation of the three fundaments takes place, as in Triton, and we then have (1) a round chordal rod (ch), which has been formed by infolding in the manner previously described ; (2) at either side of it the small-celled mass of the middle germ-layer (mk), divided into halves by the chorda ; (3) the inner germ-layer (ik), the halves of which, separated in the previous stage, are now growing under the chorda, and are about to fuse into a single layer.
+++++++++++++++++++++++++++++++++++++++++ Fig. 93. Cross section through the blastoderm of a Chick in which the first traces of the chorda and the medullary furrow are to be seen, after BALFOUR AND DEIGHTON.
The section passes through the fundament of the chorda in front of the primitive streak. The part of the section at the right of the fundament of the chorda is not figured.
ak, Outer, ink, middle, ik, inner germ-layer ; ck, fundament of the chorda.
+++++++++++++++++++++++++++++++++++++++++ A similar view is furnished by a cross section through the cephalic process of the germ of the Chick (fig. 93). Under the outer germlayer there is found in the median plane, in front of the primitive groove, only the fundament of the chorda (c/i) ; at the point indicated by a star it is continued laterally into the small-celled middle gerinlayer, and into the entoderm, which is composed of a single layer of very much flattened cells.
The same is true for cross sections of Mammals (fig. 94) in corresponding stages of development. Thus, for example, the fundament of the chorda (ch) in the cross section through the embryo of a Mole figured by HEAPE is a single layer of cylindrical cells ; it has already become curved into a chordal groove, such as has been represented in fig. 79 A for Triton. Laterally it is continuous with a mass of small cells, which is resolved into two layers at the point indicated by a star : (1) into the middle germ-layer (mk), composed of several layers of small cells ; and (2) into the inner germ-layer, which, as before, appears as a single layer of flattened cells (ik).
+++++++++++++++++++++++++++++++++++++++++ Fig. 94. Cross section through the embryonic area of a Mole which is in about the stage of the Rabbit represented in Fig. 89 B. After HEAPE. The section passes through the chordal groove (ch) somewhat farther forward than the section represented in Fig. 97, which lias encountered a region that is to be interpreted as the blastopore. ak, Outer, mk, middle, ik, inner germ-layer ; ch, fundament of the chorda. +++++++++++++++++++++++++++++++++++++++++
In a still more convincing manner VAN BENEDEN has shown, in his investigations upon the development of Mammals, that conditions exist in the formation of the middle germ-layer and of the bodycavity in this class which agree with those in Amphibia. The cross section (fig. 95) through the germ-disc of the Eabbit, taken from his work, is especially convincing. It shows the fundament of the chorda (ch) as a single layer of cylindrical cells, flanked on the right and left by the middle and inner germ-layers. The middle germlayer consists of a parietal (mk 1 ) and a visceral (ink 2 ) lamella of flat cells, the former of which is continuous with the fundament of the chorda, while the latter bends around at the point indicated by a star to become continuous with the single-layered epithelium of the inner germ-layer (ik). The place where the bend occurs even protrudes distinctly as a lip into the ccelenteron, as in the case of the Amphibia. Except for these unions at the sides of the chordal fundament, the middle germ-layer is everywhere sharply separated by a fissure from the other two germ-layers.* Further agreement with the conditions which the investigation of Triton has furnished is afforded by a series of cross sections through the primitive streak the obliterated blastopore. In the case of all Vertebrates, this is the only place in the whole embryonic area where all three germ-layers, although for only a short distance, are fused with one another, and cannot be distinguished as separate layers, whereas at the sides of this region they are separated by distinct fissures.
+++++++++++++++++++++++++++++++++++++++++ Fig. 95. Cross section through the germ-disc of an embryo Rabbit, after E. VAN BENEDEN. "k, Outer, ik, inner, mk, middle germ-layer ; ink 1 , parietal, mk~, visceral lamella of the middle genii-layer; ch, chorda.
+++++++++++++++++++++++++++++++++++++++++ Fig. 96. Cross section through the middle of the primitive streak of a Chick's germ-disc, which is in the stage of development represented in Fig. 81 B. After ROLLER.
At some distance from the primitive groove is to be seen upon the left side of the figure in cross section the marginal groove of His. Upon the right side it is as yet little developed.
ale, Outer, ik, inner, mk, middle germ-layer ; pr, primitive groove ps, primitive streak ; gr, marginal groove.
Figure 96 represents a cross section through the embryonic area of a Chick in which the primitive groove is distinctly developed,
- In the development of Mammals there has been observed at certain stages under the fundament of the chorda a peculiar structure, the so-called chordal canal, which is not found in the other classes of Vertebrates. I mention it here only incidentally, because the publication of VAN BENEDEN'S investigations will doubtless furnish the desired explanation of its origin and significance.
but in which no traces of the medullary folds are to be observed. The outer germ-layer (ak) is composed of a single layer of tall cylindrical cells, the inner germ-layer (ik) of a single sheet of greatly flattened elements. In the space between the two there penetrates at both sides of the primitive groove a mass of small cells in many superposed layers, the middle germ-layer (ink). In the region of the primitive groove (pr) this goes over continuously into the outer germ-layer, the cells of which are here found in proliferation, whereas its lateral wings are separated from, the outer layer by a fissure. The lower germ-layer is drawn by ROLLER from whose work the accompanying figure is taken as being everywhere a
Fig. 97. Cross section through the embryonic area of a Mole, which is in a stage corresponding approximately with that of the Rabbit represented in Fig. 89 B, After HEAPE. The section passes through the primitive groove, somewhat behind the one represented in Fig. 94. ale, Outer, ik, inner, mk, middle germ-layer ; it, primitive groove. +++++++++++++++++++++++++++++++++++++++++
separate sheet of flattened cells. It is clear, however, from other drawings and descriptions by DUVAL, RABL, and others, as well as from the accounts in regard to the similar development of Reptiles, that for a certain distance underneath the primitive groove the middle germ-layer is as little to be distinguished as a separate structure from, the lower as it is from the upper germ-layer.
Cross sections through the primitive groove of mammalian embryos are very instructive (fig. 97). According to HEAPE'S investigations on the Mole, the groove (u) cuts deeply into a mass of small cells. At this place all three layers are fused together ; and it is only laterally to this that they are separated by means of a distinct fissure, and that each is distinguishable by its characteristic kind of cells the outer (ak) by its tall, the inner (ik) by its much-flattened, and the middle (ink) by its small, more spherical or polygonal cells.
The conditions of the germ-disc of the Rabbit found by VAN BENEDEN are especially distinct (fig. 98). At the deep incision of the primitive groove (pr) all three germ-layers are joined to one another for a certain distance by means of a common cells mass. At the same time one may observe, with tolerable distinctness, how the outer germ-layer (ak) bends around into the parietal middle layer (mk 1 ) at the primitive fold (ul), while the visceral lamella (mk 2 ) is continuous with the entoderm (ik), which is only one cell thick. Indeed, in embryos of Rabbits and Bats, VAN BENEDEN in some cases observed between the primitive folds, or blastoporic lips, a structure corresponding to the yolk-plug of Amphibia.
+++++++++++++++++++++++++++++++++++++++++ Fig. 98. Cross section through the primitive groove (blastopore) of a Rabbit's germ-disc, after ED. VAN BENEDEN. ak, Outer, ik, inner, mk, middle germ-layer ; mk 1 , parietal, mk?, visceral lamella of the middle germ-layer ; ul, lateral lip of the blastopore ; -pr, primitive groove. +++++++++++++++++++++++++++++++++++++++++
+++++++++++++++++++++++++++++++++++++++++ Fig 99. Cross section through a human germ-disc, with open medullary groove, in the vicinity of the neurenteric canal (jir), after GRAF SPEE. ak, Outer, ik, inner germ-layer; mk 1 , parietal, mk~, visceral lamella of the middle germ-layer; ul, lateral lip of the blastopore ; pr, primitive groove.
It is certainly of great general interest that the investigation of an extraordinarily young human germ-disc at the hands of GRAF SPEE has furnished a cross section (fig. 99) which is near enough like the one of the Rabbit here figured to be mistaken for it. In the case of the human embryo, one sees a deep-cutting primitive groove, and at the easily recognisable blastoporic lip (id] the bending over of the outer germ-layer (ak) into the parietal lamella (mk l ). The visceral lamella (7?t& 2 ) is well separated from the latter for some distance ; under the primitive groove it is merged \vith the inner germ-layer, the edges of the potential folds of the two sides being fused into a mass of cells, which forms the floor of the primitive groove.
Finally an agreement with the development of the Amphibia is not wanting in sections which are made through the embryonic areas of Birds, Reptiles, and Mammals behind the primitive groove. The middle germ-layer begins to spread itself out backward also, not, however, as in the anterior part of the embryonic area, in the form of paired fundaments, but rather as a single continuous cell-mass. This outgrowth too is united to the two primary germ-layers only in the region of the posterior end of the primitive streak, being elsewhere distinctly separated from both of them.
For the completion of the previous account, some statements about the further growth of the middle germ-layer may now be added, concerning which cross sections through embryos of various ages afford evidence. The middle germ-layer spreads itself out on all sides between the two primary germ-layers, farther and farther from the place of its first formation the vicinity of the primitive groove. At first it is limited to the fundament of the embryo itself, then it makes its way into the area pellucida, and, finally, it is encountered in the opaque area. Everywhere and constantly in its extension it appears as an entirely independent layer, at least two cells thick, which is separated from its surroundings by fissures. It is found to be united for a short distance with the inner and outer germ-layers, but only at the primitive groove, which persists for a long time, in older embryos even, as we havo already learned from surface-views. Even in the stage when the neurenteric canal traverses the primitive streak, and puts the ccelenteric cavity (under the entoderni, fig. 100 hy) in communication with the neural tube, we -see the cellular lining of the canal and the middle germ-layer fused, so that in this region a connection still exists between all three germinal layers. Compare the accompanying cross sections through embryos of Lacerta nmralis.
After the statement of the actual conditions, the questions remain to be answered : (1) What is the meaning of the primitive groove ? (2) How is the middle germ-layer developed 1 In the interpretation of the primitive groove I place myself, as is to be seen from what precedes, wholly on the side of those investigators who, like BALFOUR, HATSCHEK, KUPFFER, HOFFMANN, VAN BENEDEN, L. GERLACH, RUCKERT, and others, recognise in it a structure equivalent to, but somewhat modified from, the blastopore of lower Vertebrates, and who compare the primitive Jolds to lateral blastoporic lips closely pressed together. In my description of a previous stage I have already designated as blastopore the crescentic groove of Birds (fig. 52 B s) and the prostoma (fig. 55 u) of Reptiles, because that is the place w T here the lower germ-layer is infolded. In my opinion both grooves are identical structures, which, by changes in position and form, have been so evolved, the one from the other, that the fissure, -which was at first transverse, has become converted into a longitudinal one. For Reptiles KUPFFER has established this to a certainty. According to his figures in Ernys Europsea, e.g., the transverse depression (u) represented in fig. 101 A is converted at a later stage into the form shown in the adjacent figure (101 B u). For the Birds the investigations of DUVAL previously recounted (p. 121, fig. 82) are convincing. There is also to be taken into account, the additional fact, that even as early as in the Amphioia an exactly corresponding metamorphosis of the blastopore takes place. As the accompanying cuts (fig. 101 C and D) show, the blastopore of the Amphibian is, at its first appearance, a transverse fissure (fig. 101 C u). Then it becomes circular, and embraces with its lips a protruding portion of the otherwise enclosed yolk-mass, the yolk-plug, becomes narrower, and is continued forward into a longitudinal groove. Finally it appears (fig. 101 D u) as a deep groove, situated at the end of the medullary furrow, with its small circular opening filled up with a yolk-plug.
+++++++++++++++++++++++++++++++++++++++++ Fig. 100. Cross sections through the posterior end of a young embryo of Lacerta muralis, after BALFOUR.
In figure A the neurenteric canal is cut lengthwise ; in figure B only an evagination of it, which is directed backward. Since the sections pi'obably have not cut the chief axis of the embryo perpendicularly, the middle germ-layer is fused with the wall of the canal only on the right side in figure A, whereas in figure B the connection is present on both sides.
ne, Neurenteric cana ; ep, outer, mejt, middle, hy, lower germ-layer.
In addition there are three important considerations which may be urged in support of the interpretation of the primitive groove as blastopore.
First, the primitive streak, even when an open canal is wanting, is the only place in the whole germ-disc where a connection between all the germ-layers is constantly present, as at the Amphibian blastopore.
+++++++++++++++++++++++++++++++++++++++++ Fig. 101. A and B. A portion of a younger and of an older embryonic fundament of Emys Europsea, with the prostoma or blastopore (), after KUPFFER. ul, Lip of the blastopore. C and D. Two eggs of Triton taeniatus seen from the blastopore, one 30 hours, the other 53 hours after artificial fertilisation. u, Blastopore ; h, elevation between blastopore and dorsal groove ; /, semicircular furrow, which encloses the blastoporic area ; dp, yolk-plug. +++++++++++++++++++++++++++++++++++++++++
Secondly, the chief organs of the body, such as the chorda, the neural tube, and the primitive segments, are developed in front of the primitive streak in the case of the higher Vertebrates, just as they arise in front of the blastopore in Amphioxus and the Amphibia. Both blastopore and primitive streak occupy the posterior end of the body. The so-called cephalic process of the primitive streak is nothing else than the first rudiment of the chorda.
Thirdly, one may still recognise in the openings canales neurenterici which have been pointed out in the primitive streak at an earlier or later stage in its development, in the case of Birds, Reptiles, and Mammals, an indication that an open communication has existed here from the beginning between the inner and the outer germ-layers ; further, that this communication has disappeared through the fusion of the blastoporic lips, but that it can be in part reestablished in consequence of more favorable processes of growth. At the same time the neurenteric canal, in cases where it reappears in the primitive streak, effects a very characteristic union between the posterior ends of the neural and intestinal tubes, in exactly the same manner in which the blastopore of Amphioxus, the Amphibia, and the Selachii does (compare fig. 80 with fig. 88 n.e).
In the interpretation of the primitive groove as blastopore I am compelled to oppose a somewhat different view. Certain investigators (BALFOUR, RAUBER, and others) recognise in the primitive groove and the crescentic groove of meroblastic eggs only a small part of the blastopore ; they interpret as the major part of it the region which is encircled by the whole rim. of the germ-disc and is occupied by the yolk-mass, and to which they give the name yolkblastopore.* According to their conception, as also according to the original assumption of HAECKEL, the two-layered germ-disc is a flattened-out gastrula, its blastoporic rim lying upon the yolksphere, which gradually grows around the yolk, and finally takes the latter wholly inside itself, just as if it were a ball of food. The primitive groove is a small detached part of the blastopore, which is connected with the development of the middle germ-layer. The two parts become completely separated from each other, and are closed at different times, each for itself, the yolk-blastopore often late, at the pole of the yolk-sac which is opposite to the embryo.
Such an assumption of a double blastopore appears to me to be untenable. / propose that only that place, of the germ be designated as blastopore at which, as in the gastrulation of Amphioxus and the Amphibia, there actually occurs an invagination of cells, by means of which the cleavage-cavity is obliterated. Such a process takes place in the Selachii only at the crescentic hinder part of the margin of the germ-disc, in the Reptiles and Birds at the small place designated as crescentic groove. It is also from this place alone that subsequently the development of the middle germ-layer proceeds.
The anterior margin of the germ-disc in Selachians, and, after the conversion oj the crescentic groove into the primitive groove, the tvhole
- RAUBER has suggested for the various regions which he assumes for the blastopore the designations prostoma sulcatum longitudinals (primitive groove), prostoma sulcatum falciforme (crescentic groove), and prostoma mar g indie (yolk-blastopore).
margin of the germ-disc in Birds and Reptiles, have an entirely different signification. This margin exhibits a very different relationship from that of the primitive streak or blastopore ; it is a peculiarity of meroblastic eggs, which is most intimately associated with the origin of partial cleavage. It indicates the place at which the segmented portion of the germ meets the unsegmented portion the place at which there lie in the yolk free nuclei, by means of which a supplementary cleavage is kept up until late stages in the process of development, until, in fact, the time when the two primary germlayers have been formed by means of the invagination which occurs at the blastopore. At the expense of the cell-material, which is constantly being augmented by supplementary cleavage, the germlayers increase in extent at their place of transition into the yolk, and thus gradually grow over the unsegmented part. Whereas at the blastopore an invagination of cells already present takes place, there ensues at the margin of the germ-disc a formation of new cells, and thereby an increase of the marginal part and an overgrowth of the yolk. I therefore propose for it the name circumcrescence-margin of the yolk-sphere. There can be no such thing as a separate opening or a yolk-blastopore, because the yolk is an organic part of the germ, and is in continuity with the segmented part of it by means of the layer which contains the yolkxiuclei.
If we would institute a comparison between animals with meroblastic eggs and the Amphibia at a stage when gastrulation is not yet completed, then the blastopore of the Amphibia, which is indicated by the letter ^l in tile accompanying section through the gastrula of a Triton (fig. 102), corresponds to the prostoma of Reptiles, and to the crescentic and primitive grooves of Birds ; the still exposed mass of yolk-cells corresponds to the yolk-material which is not yet overgrown by germ-layers ; the place marked by a star, at which in the Amphibia the transition from the small-celled layer to the mass of yolk-cells occurs, or the marginal zone of GOETTE, is comparable to the margin of circumcrescence in meroblasbic egg
+++++++++++++++++++++++++++++++++++++++++ Fig. 102. Longitudinal section through a gastrula of Triton. ak, Outer, ik, inner germ-layer ; fh, cleavage-cavity ; ud, coel enteron ; u, blastopore ; dz, yolk-cells ; dl, dorsal, vl, ventral lip of the coelenteron.
In the second place, the question arises : How is the middle yermlayer of Vertebrates developed ? The answer is : By a process of folding similar to that in the case of Amphioxus lanceolatus. This answer is substantiated by the fact that the individual processes in the development of the middle germ-layer may be correlated with corresponding processes in Amphioxus.
In view of the fundamental importance of the matter, I formulate in a synoptic and precise manner in six paragraphs the points in reference to which it has been possible to establish an agreement in all Vertebrates.
1. Before the chorda is formed, the germ in all Vertebrates is composed of two layers in the region of a median streak which lies in front of the blastopore and primitive groove. It is here composed of the medullary plate and the fundament of the chorda, which then shares in bounding the intestinal cavity.
2. At both sides of this median streak the germ is three-layered, if we regard the middle germ-layer as a single one ; it is four-layered, if we allow that the latter consists of a parietal and a visceral celllayer, which are originally pressed firmly together, and only later actually separated by the appearance of the body-cavity.
3. In 110 Vertebrate do the middle germ-layers arise by fission, either from the outer or the inner germ-layers, because they are everywhere, except in a very limited region of the germ, sharply separated from both by means of a fissure.
4. A connection of the middle germ-layers with the neighbouring cell-layers takes place only : (a) at the blastopore or primitive groove, where all four (or three) germ-layers are joined together, and (b) at both sides of the fundament of the chorda.
5. One observes the first fundament of the middle germ-layers at the region of the germ just mentioned, and sees it spread itself out from here i.e., from the periphery of the blastopore or the primitive groove, and from both sides of the fundament of the chorda forward, backward, and ventrad or laterad. In front of the blastopore it appears in the form of paired fundaments separated by the fundament of the chorda ; behind the blastopore, on the contrary, as a continuous structure.
6. While the chorda is being developed, the two paired fundaments of the middle germ-layers detach themselves from the adjacent celllayers at the sides where their ingrowth took place, and at the same time the halves of the permanent entoderm grow together, whereby the dorsal closure of the intestine is effected.
In view of these facts there is only one explanation at which we can arrive. If it is certain that the middle germ-layers do not arise by a fission in loco from either of the primary germ-layers, then their gradual spreading out from a definite region of the germ can result only from an ingrowth of cells, which occurs from those places where a connection with other cell-layers has been demonstrated. The middle germ-layers draw the principal material for their growth from cells which, at the blastopore or at the primitive groove, migrate between the two primary germ-layers.
But this immigration of cells may be interpreted as a process of infolding of the primary germ-layers, as in the case of Amphioxus. In the method of the infolding there exists, it is true, one very striking and apparently important difference between Amphioxus and the remaining Vertebrates. In Amphioxus the middle germlayer arises as a hollow sac, by means of the folding of the inner germ-layer in the remaining Vertebrates as a solid mass of cells. This undeniable difference is, however, easily explained in the following manner : In the solid fundaments of the middle germlayer a cavity is wanting, because the cellular walls of the sac are from the beginning firmly pressed together, in consequence of the yolk-mass which fills the coalenteron. In addition to other striking agreements with the conditions in Amphioxus lanceolatus, there are three pointsof viewwhich in particular com mend this interpretation : (1) In all vertebrated animals there early arises in the middle germ-layer a fissure, which is surrounded by cells, often cubical or cylindrical, having an epithelial arrangement. The parietal and visceral layers then take the form of epithelial lamellae, as is to be seen in an especially striking manner in the case of the Selachii at a very early stage of development. (2) From these epithelial layers there arise in the adult genuine epithelial membranes, like the ciliated peritoneal epithelium of many Vertebrates, and, in addition, glands that in many respects resemble the glands derived from, epithelial membranes [of the other germ-layers] (kidney, testis, ovary). (3) The objection that the middle germ-layer of Vertebrates arises as a single cell-mass, and therefore cannot be equivalent to two layers of epithelium, loses its weight with every one who knows the numerous analogous phenomena of development occurring elsewhere, in which organs that should be hollow are at first developed as solid masses of cells. We shall hereafter cite as such the solid fundament of the neural tube in Bony Fishes, many sensory organs and the most of the glandular sacs, which latter arise as solid buds of epithelial lamellae, and only later, when they become functionally active, acquire a cavity by the separation of their cells.
1. Out of the mass of cleavage-cells (morula) there is developed in all Vertebrates a sac-like germ (blastula) with cleavage-cavity.
2. There are four different kinds of blastulse in Vertebrates, according to the amount and distribution of yolk.
(a) In Amphioxus the cleavage-cavity is very large, and its wall consists of a single layer of cylindrical cells of nearly uniform size.
(b) In Cyclostomes and Amphibia the cleavage-cavity is small : one half of the wall of the blastula is thin, and composed of one or several layers of small cells ; the other half is considerably thickened, and formed of large yolk-cells arranged in many superposed layers.
(c) In Fishes, Reptiles, and Birds (rneroblastic eggs) the cleavage-cavity is small and fissure-like or wanting. Only its roof or dorsal wall consists of cells (germ-disc) ; its floor or ventral wall, on the contrary, consists of the yolk-mass which has not been divided into cells, but which contains yolk-nuclei in the vicinity of the margin of the germ-disc.
(d) In Mammals the cleavage-cavity is very spacious, and filled with an albuminous fluid ; its wall is composed of a single layer of greatly flattened hexagonal cells, with the exception of a small thickened place, where larger cells in several superposed layers cause an elevation which projects into the cavity.
The cup-shaped larva or gastrula with two germ-layers
1. There is formed out of the blastula, by the invagination of a portion of its surface, a two-layered form, the beaker-larva or gastrula.
2. The two layers of the double beaker are the outer and the inner germ-layer (ectoblast, entoblast) ; the fissure separating the two layers is the obliterated cleavage-cavity; the cavity resulting from the invagination is the ccelenteron, its external opening the primitive mouth (blastopore, prostoma, crescentic groove, primitive groove).
3. The four kinds of gastrulse correspond to the four kinds of blastulse.
(a) In Amphioxus the cceleiiteron is wide, and each germ layer is made up of a single sheet of cylindrical cells.
(b) In Cyclostomes and Amphibia the mass of yolk-cells is accumulated on the ventral wall of the ccelenteron in the inner germ-layer, and causes a protuberance, by means of which the ccelenteron is reduced to a fissure.
(c) In Fishes, Eeptiles, and Birds the process of invagination remains confined to the germ-disc, since the unsegmented yolk, on account of its considerable volume, cannot be made to share in the invagination. The germ-disc becomes two-layered by means of an ingrowth of cells at the crescentic groove (blastopore). The yolk acquires a cellular boundary very slowly and at a late period ; it is overgrown by the margin of the germ-disc, when the supplementary cleavage (yolk-nuclei) takes place.
The outer germ-layer spreads itself out and envelops the yolk most rapidly ; then follows the inner, and finally the middle layer.
(d) In Mammals the inner germ-layer is developed from the thickened region of the blastula, probably by means of an invagination, because at a later stage an orifice of invagination, comparable with the primitive groove of Birds, or a blastopore, can be demonstrated. At the beginning of its development the inner germ-layer terminates below in a free margin, so that the ccelenteron is for a time closed in on the ventral side by the o ater germ-layer only, a peculiarity which is comparable with the conditions in Reptiles and Birds, if we conceive the yolk-material to have disappeared in this instance before it is completely surrounded by the inner germlayer.
4. In Vertebrates the gastrula presents a sharply expressed bilateral symmetry, so that one can easily distinguish the future head- and tail-ends, the future dorsal and ventral sides of the body. The blastopore (crescentic groove, primitive groove) marks the posterior end. The ventral side is characterised by being the place where the segmented or unsegmented yolk-material comes to lie.
The embryo with four germ-layers and a body-cavity
1. In all Vertebrates there are formed from the roof of the ccelenteron two lateral evaginations of the inner germ-layer, by means of which the ccelenteron is divided into a median cavity, the secondary intestine, and two lateral cavities, the two body-sacs.
2. The primary inner germ-layer is resolved in consequence of this process of evagination into three parts :
First, the epithelial lining of the intestinal tube (secondary inner germ-layer Darmdriisenblatt).
Secondly, the epithelial lining of the body-cavity, or the middle germ-layer, in which a parietal and a visceral layer are distinguishable. Thirdly, the chorda, which takes its origin from the portion of the primary inner germ-layer which lies between the lateral evaginations from the roof of the coelenteroii.
3. Two modifications of the process of evagination can be recognised in the case of Vertebrates.
(a) In Amphioxus the evaginations are small, numerous, and segmentally arranged ; provided from the first with a cavity ; and, beginning in the fundus of the ccelenteron, developed toward the blastopore.
(b) In the remaining Vertebrates, instead of hollow sacs, there grow out from the inner germ-layer two solid masses of cells : (1) In the vicinity of the blastopore (primitive groove, peristomal mesoblast).
(2) From here forward along the roof of the crelenteron, at a slight distance from the median plane, at both sides of the fundament of the chorda (gastral mesoblast).
The paired fundaments spread themselves out from their place of origin between the two primary germlayers farther forward and ventralward.
4. The three organs derived from the primary inner germ-layer (middle germ-layer, fundament of the chorda, secondary inner germlayer) are separated from one another by constrictions.
First, the body-sacs are detached from the fundament of the chorda and the entoblast, whereupon the edges of the parietal and visceral lamellee, thus set free, fuse with each other.
Secondly, the fundament of the chorda is bent into a chordal groove, and this is converted into a solid rod, which is completely isolated from the entoblast.
Thirdly, the entoblast closes together into a tube with a dorsal raphe.
5. The development of the three fundaments, as also that of various other organs, begins at the head-end of the embryo, a] id advances from here toward the blastopore, where for a long time a continual formation of new parts and an increase in the longitudinal growth of the body take place.
6. During the development of the middle germ -layer, the blastopore of the Amphibians, Fishes, Reptiles, Birds, and Mammals has been metamorphosed into a groove occupying the longitudinal axis of the embryo (primitive groove of the higher Vertebrates).
7. The blastopore and the primitive groove in later stages of development undergo degeneration, and are not converted into any organ of the adult. (For the details of this, see Part II.)
8. Before their disappearance the blastopore and primitive groove are surrounded by the medullary folds and taken into the terminal part of the neural tube, whereby a direct communication between neural tube and intestinal tube the neurenteric canal is effected. The two organs, which communicate with each other for a long time, are later separated by its closure.
Text-Book of the Embryology of Man and Mammals: Description of the Sexual Products | The Phenomena of the Maturation of the Egg and the Process of Fertilisation | The Process of Cleavage | General Discussion of the Principles of Development | The Development of the Two Primary Germ-Layers | The Development of the Two Middle Germ-Layers | History of the Germ-Layer Theory | Development of the Primitive Segments | Development of Connective Substance and Blood | Establishment of the External Form of the Body | The Foetal Membranes of Reptiles and Birds | The Foetal Membranes of Mammals | The Foetal Membranes of Man | The Organs of the Inner Germ-Layer - The Alimentary Tube with its Appended Organs | The Organs of the Outer Germ-Layer | The Development of the Nervous System | The Development of the Sensory Organs | The Development of the Skin and its Accessory Organs | The Organs of the Intermediate Layer or Mesenchyme | The Development of the Blood-vessel System | The Development of the Skeleton
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