Book - Vertebrate Embryology (1949) 3

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McEwen RS. Vertebrate Embryology. (1949) IBH Publishing Co., New Delhi.

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Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding.     (More? Embryology History | Historic Embryology Papers)

Part III The Teleosts and Gymnophiona

The Teleosts and Gymnophiona: Their Segmentation and Gastrulation

Before beginning the study of the Chick, it is desirable to give a very brief account of the processes of segmentation and gastrulation in the Teleosts (Bony Fishes) and the Gymnophiona. It is of advantage to understand these processes in the forms mentioned because embryologicall-y they are intermediate between those found in the Frog and those in the Reptile or Bird, i.e., the Sauropsids. This of course is not meant to imply that modern Fishes, Amphibians, and Sauropsids form a direct phylogenetic series. It is merely suggestive in a general way of the manner in which the embryological conditions in the lower forms have apparently been modified in the process of evolution.

The Teleosts Segmentation

In the Frog the yolk is more or less concentrated in the vegetal half of the egg, but is not sufliciently dense to prevent the whole egg from segmenting. In the Teleosts, on the contrary, the concentration of yolk is very marked, so that the protoplasm exists only as a thin plate upon the animal pole. As noted in Chapter II, this plate is called the blastodisc, and from it the entire embryo arises, the remainder of the egg being purely nutritive. In these eggs, therefore, when segmentation begins, the process is confined to this disc, and is said to be meroblastic or discoidal, as opposed to the holoblastic or total cleavage of Amphioxus and the Frog (Fig. 136).

The first two planes of division pass entirely through the disc and at right angles to one another. Normally the third cleavage is at right angles to the second, so that at this point the pattern may be described as bilateral with respect to the plane of the latter cleavage. This feature is further emphasized in many Teleost eggs by the temporary lengthening of the blastodisc along the axis of this second plane. Thus instead of being circular at this stage the disc is an oval (almost an oblong), its long axis commonly consisting of two rows of four cells each. The fourth cleavages then generally come in at right angles to the first so that we have four rows of four cells each, two on either side of the second cleavage plane, i.e., on either side of the long axis of the oval (Fig. 137, C). However, shortly after this the dividing blastodisc ceases to be

Fig. 136. Egg of the Teleost, F undulus heteroclitus. From KelIicott (General Embryology). Total view, about an hour after let'tilization. c. Chorion. d. Protoplasmic germ disc or blastodisc. 0. Oil vacuoles. p. Perivitelline space. 11. Vitelline membrane. 9'. Yolk.

an oval and again becomes circular. Further cleavages ensue, and the disc is thus presently transformed into the blastoderm. This consists of small cells whose original relationships are impossible to trace unless vital stains or other means of identification have been employed. Between this blastoderm and the yolk, a space has meanwhile developed.

which is termed the segmentation cavity, and which corresponds to the L cavity of the same name in the Frog (Fig. 137, D, E). Thus the egg has become a blastula.

In the yolk around the margin of the blastoderm are a number of nuclei (yolk nuclei) derived partly from the blastoderrnal edge, and f’ partly perhaps from extra sperm (merocytes). These nuclei presently I T begin to divide amitotically, and become amoeboid, in some cases migrating centrally beneath the blastoderm. Here they occupy the thin layer of protoplasm forming the floor of the segmentation cavity, which thus has the character of a syncytium. This syncytium or periblast, as it is termed, presently spreads over the entire yolk, and is perhaps instrumental in making the latter assimilable by the cells of the blastederm. At all events, it finally disappears without taking any part in the formation of the actual embryo; hence it need not be considered further.

Fig. 137.—Cleavage in the Sea-bass, Serranus atrarius. From H. V. Wilson. A. Surface view of blastoderm in two-cell stage. B. Vertical section through four-cell stage. C. Surface view of hlastoderm of sixteen cells. D. Vertical section through sixteen-cell stage. E. Vertical section through late cleavage stage.

Central periblast. m.p. Marginal periblast. s.c. Segmentation cavity (blastecoel)

There have been several attempts to discover what determines the antero-posterior axis in the Fish, but none in the writer’s opinion has been very successful, including his own. It is a fact that in the forms which have been studied this axis usually coincides with the second plane of cleavage. But this is not always so, and what causes the variation no one really knows. Whatever the determining factor or factors may be the axis becomes manifest with the advent of gastrulation.

Involution. — In that region of the blastoderm which is destined to form the posterior end of the animal, the blastodermal rim begins to turn under, i.e., is involuted. Thus, in this region a lower layer of cells begins to spread anteriorly into the segmentation cavity beneath the blastoderm. It is the hypoblast, destined later to give rise to the endoderm, notochord and rnesoderm, while the remaining upper layer is the epiblast. The margin of the blastoderm where the involution is occur arche ceronring, constitutes the dorsal blastoporal lip, while the former segmentation cavity now lying between the spreading hypoblast and the yolk is the archenteron, (Figs. 138, 139). The new cavity, like its predecessor, is obviously extremely shallow, and though roofed by the hy poblast is open below to the surface of the yolk, or more strictly speaking to the thin syncytial layer of periblast. Lastly, it is to be noted that while the process of involution is most active at the posterior edgeof the blastederm, it is also occurring to a much lesser degree all around the margin.

Fig. 138. — Diagram of a median sagittal section of a Teleost gastrula shortly before the closure of the blastopore. From H. V. Wilson, slightly modified.

Epiboly. While involution is thus progressing chiefly at the posterior edge of the blastoderm, very active epiboly is taking place about the remainder of the blastodermal margin, the result being to envelop the entire yolk with an epiblastic covering of cells, the yolk-sac, and concurrently to close the blastopore. In such cases, as suggested in Chapter II, it is possible to regard the entire rim of the growing blastederm as the blastoporal lip. Thus while the posterior edge is recognized as the dorsal lip, the lateral edges must be regarded as the lateral lips and the anterior edge as the ventral lip. It may be noted, furthermore, that in some forms, e.g., Serranus, the Sea Bass, according to Wilson (’89) , the epibolic process is most rapid‘ at the anterior edge (ventral lip), and decreases along either side until at the posterior edge (dorsal lip) it is comparatively slight. Under such circumstances the above homology is particularly obvious because, owing to its relatively rapid growth, the anterior edge passes clear around the vegetal pole and up on to the posterior side, thus becoming actually a ventral lip in position as well as in name (Figs. 138, 14-0) . How widespread among Fish eggs this characteristic of the relatively excessive growth of the anterior edge of the blastoderm may be cannot be definitely stated, because in most descriptions the point is not made clear. This is due partly perhaps to difficulty in many cases of being sure of the constant orientation of the parts of the egg, which in the Sea Bass is said to be fixed by the position of the oil globule. In at least one other instance, however, i.e., that of the oval egg of Hemichromis (McEwen ’30) , this orientation is equally well or better established by the shape of the egg. In this case the blastoderm is at one end of the oval, and the egg does not normally turn end over end within its chorionic membrane because of the stiffness of the latter and its own viscosity. It is thus possible to observe that epiboly, unlike that in Serranus, is clearly equal on all sides. Hence the blastopore obviously closes on exactly the opposite side (end) of the egg from the original animal pole (Fig. 141).

Fig. 139. Sagittal sections through the blastoderm of Serranus during the formation of the germinal layers. From Jenkinson (Vertebrate Embryology). After H. V. Wilson. A. Beginning of involution and slight epiboly at dorsal lip (d.l.) B. Epiboly at anterior edge. C Further progress of involution at dorsal lip.

d.l. Dorsal lip. par. Parablast (periblast).

Fig. 140. - Growth of the blastoderm over the yolk (epiboly) after the formation of the material for the embryo in the Teleostan fish Serranus. From Jenkinson (Vertebrate Embryology). After H. V. Wilson. A Lateral view of a very early stage of gastrulation. B. Dorsal view of a much later stage. C. Lateral view of the same stage as B. D. Lateral view of a late stage, gastrulation almost complete.

a’.l. Dorsal lip of the hlastopore (posterior edge of the blastoderm). a.e. Anterior edge ofthe hlastoderm or ventral lip (v.l.) of the blastopore. s.c. Segmentation cavity. o.g. Oil globule.

Concrescence or Conve1'.gence.—The Fish, as previously stated, is.the form in connection with which the theory of concrescence originated, and it may be that this process does occur here to a limited extent. However, as in other cases, it is now considered that the movement which takes place in this form is more aptly designated as convergence _ (Oppenheimer, ’36). It goes on of course along with the epiboly, and seems to involve two things. There is on the one hand some actual con cresence or confluence of material in the germ ring on either side of the dorsal lip of the blastopore. The greater part of this material, however, flows more directly, partly toward the lip and partly toward the median line, i.e., it converges toward these regions (Fig. 14-2) . This and the involution leads to a piling up of cells in a somewhat shield shaped area anterior to the dorsal blastoporal lip, the base of the shield being adjacent to the lip. This area is in fact known as the embryonic shield, and it is along its median longitudinal axis that the outline of the embryo presently appears as indicated in Fig. 141, C.

Fig. 141. A and B early stages, C and D, late stages in the gastrulation of the Teleost. Hemichromis bimaculata. A and C are dorsal views, B and D are lateral views. Note the equal epibolic growth of the blastoporal lips, unlike the condition in Serranus.

Meanwhile as the lips of the blastopore finally close posterior to the shield they leave, at least in some embryos (Sea Bass, H. V. Wilson), a short thickened line of cells. At the anterior end of this line is a slight cavity extending upward from the shallow archenteron (Figs. 138, 143) .

It is called Kupfier’s vesicle, and seems to be an incipient neurenteric canal. It cannot be a genuine neurenteric canal since the nerve cord, because of its peculiar method of formation in the Fish, does not yet itself possess a lumen. At the posterior end of the thickened line is the place of final blastoporal closure, and probably also the place where the future anus opens. However, since the Fish unlike the Frog does not have a proctodael invagination to mark this spot, the latter point is not certain. Assuming, nevertheless, the homology of Kuplfer’s vesicle with a neurenteric canal, and the place of blastoporal closure with the anus, the thickened line is evidently the homologue of the primitive streak of the Amphibian. On this basis it may be so designated. The mass of cells in and around the more posterior portion of it, however, because of their character and future history, are often designated as the caudal knob. Thus is produced, the Teleostean gastrula.

Fig. 142. A diagrammatic representation of the process of convergence, and incidentally a small amount of involution, essentially as they are thought to occur in the Teleosts, as well as in some other forms. A. Surface view of the blastoderm at the beginning of the processes. 3. A similar view near the completion of gastrulation. Changes in the positions of the letters and the directions of the arrows represent the movements which are supposed to have occurred. Dotted letters and arrows indicate regions which have been involuted underneath the margin, and hence would be invisible from above.

The Differentiation Of Mesoderm, Notochord, and Definitive Endoderm

It has been stated that involution occurred chiefly at the dorsal lip of the blastopore. The result is that in the region anterior to this lip, i.e., the region of the embryonic shield, the roof of the archenteron consists of an extensive double layer of cells produced by this process. From the dorsal side of the lower or involuted of these two layers (hypoblast), between it and the overlying epiblast, the mesoderm is now delaminated in two sheets situated upon either side of the middle line (Fig. 144). Presently, also, the hypoblast along the middle line itself becomes separated from that upon either hand, and is aggregated into an axial rod, the notochord, with the sheets of mesoderrn upon each side of it (Figs. 144, 145). What remains of the original hypoblast may now be spoken of as endoderm, destined to form the lining of the gut. Since, however, the formation of the notochord consumed all of the primordial cells along its line of origin, the definitive endoderm consists for a short time of two separate lateral sheets. Shortly, these sheets unite with one another beneath the notochord, and the enteric roof is thus again complete. The uppermost layer may now of course be designated as ectoderm.

Fig. 143. Sagittal section through the hinder end of a Fish ern— bryo (Serranus), showing the undifferentiated primitive streak, an terior to which the structures of the embryo are being differentiated. From H. V. Wilson.

a.p. (v.l.). Anterior margin of the blastoderm or ventral lip of blastopore, after having grown entirely around the yolk mass. bl. Blastopore. ec. Ectoderm. en. Endoderm. g.r. Germ ring. k.v. Kupfer’s vesicle. nc. Notochord. nr. ch. Nerve cord. p. Periblast. pp. (zl.l.). Posterior margin of blastoderm (dorsal lip of blastopore). pr. str. Primitive streak.

Considerations Concerning The Ultimate Origins Of These Layers

It remains to be noted that although the involution of the hypoblast comprising potential endoderm, mesoderm and notochord, occurs chiefly at the dorsal blastoporal lip, the material for these layers does not all originate here. As in the case of the Frog we have seen that about this region there exists a process of convergence whereby materials anterior and lateral to the lip are carried toward it before they are involuted to the interior. The pregastrular locations of the different components of this hypoblast are indicated somewhat diagrammatically in Figure 146 taken from Oppenheimer’s studies on Fundulus. Her conclusions were reached both by various grafting experiments and, as in the Amphibia, by marking with vital stains. From them it appears that at least a considerable part of the mesoderm and endoderm of the Fish embryo is derived from the posterior third or so of the blastoderm and from its margins.

Oppenheimer has also confirmed earlier work of a different sort by Stockard to the effect that the more anterior parts of the blastoporal lip have capacities which are not normally realized. Thus any part of the blastedermal margin if cut out and implanted into the embryonic shield may differentiate into a variety of structures which it would never form in its V

Fig. 144. Transverse sections through the difusual location‘ This may ferentiating blastoderm of Serranus showing difsuggest an inductive effect ferentiation of the roof of the archenteron into an the transplanted by the Substance Of Jenkinson (Vertebrate Embryology). After H. V.

the shield. It also may mean that the material in various parts of the margin possesses inherent potentialities which are normally inhibited as this material is involuted over the dorsal blastoporal lip (Oppenheimer, ’38). To this limited extent therefore the blastodermal margin (entire lip of the blastopore) may still be thought of as containing potentially the germ of any part, or all, of an embryo. Hence in this highly modified sense the use of the term germ ring as applied to this margin may still be justified. Finally, in connection with matters pertaining to pregastrular materials, Oppenheimer (’36) finds that blastoderms removed from the yolk and periblast previous to the 16-cell stage fail to gastrulate. Instead they behave somewhat like the upper quartet of cells from a Triton 8-cell stage which have been isolated from the lower four cells containing the gray crescent. For this reason this investigator suggests that perhaps the periblast of the 16-cell Fundulus contains a substance which influences the later destinies of these cells, but which has not previously had time to act. There is thus the implication that perhaps this periblastic substance has an organizing effect somewhat comparable to that which occurs in the gray crescent region of the Amphibian.

Early Formation Of The Embryo

As soon as the germ layers are formed in the embryonic region of the blastoderm, and while the remainder of the latter is still in the process of enclosing the yolk, the outlines of the embryo begin to become clearly evident. This is the result of a folding ofi7 process by which the embryo is gradually raised above the surface of the yolk. It is to be noted that although these procedures are fundamentally similar to what will presently be described in the Bird, there is one important difference. In the lat— ter, in spite of the constriction of all three layers beneath the embryo due to the folding off, all three nevertheless take part in enclosing the yolk mass. In the Fish on the other hand the folding ofi of the endoderm is quickly completed to form a closed tube, the primitive gut. Hence only the ectoderm and mesoderm constitute the rather wide yolk stalk, and the covering of the yolk, the yolk-sac (Figs. 144, 14-7). Aside from this difference further early development of Fish and Bird is generally similar. By virtue of the folding, accompanied by rapid growth in all directions, the embryo soon comes to extend outward above the yolk-sac which is attached like a bag to its ventral side. The tail in the Fish, it may be noted. is largely formed by outgrowth from the caudal knob.

Fig. 145. - Formation of the gut (al.e.)

in Serranus by the bending down of the sides of the roof of the archenteron. In A note also the nerve cord forming by a solid invagination of ectoderm (characteristic of many Teleosts) instead of by folds. From Jenkinson (Vertebrate Embryol ogy). After H. V. Wilson.

s.n.ch. Sub-notochordal rod. end. Endo derm.

THE GYMNOPHIONA SEGMENTATION

Segmentation in these somewhat aberrant Amphibians is again virtually meroblastic, and hence results in the formation of a blastula with T,

a blastoderm. and segmentation cavity very similar to that of the Teleost. In this case, it is true, there is a slight superficial cleavage in the yolk which forms the floor of the cavity, and this also extends out around the periphery of the blastoderm. The burl: of the yolk nevertheless, remains undivided.

Gastrulation

Involution and Epibo1y. — The advent of gastrulation becomes evident by the occurrence of involution and epiboly at what proves to be the posterior edge of the blastoderm. i.e., the dorsal blastoporal lip. As an obvious result of the involution there are presently produced the usual two layers of cells. The outer is the epiblast beneath which the inner hypoblast spreads out within the segmentation cavity above the partially segmented yolk. The method is made~evident by reference to the median longitudinal, sections of the Fig. 14-6.——A diagram of an early Teleost blastoderm in Figure 148, A and B. Up to this point, it will be noted, the movements observed are not essentially

(Fundulus) blastula. After Oppenheimer. The cells have been numbered for identification purposes in discussion of subsequent stages by the author, but are not pertinent to the account in this text. The point to be noted here is the location at this stage of the areas which will later form nervous system (vertical hatch diflerent from those which ing), notochord (heavy stipple), endoderm took place at a corresponding stage in the Fish. The hatching) point in which the gastrulation of the Gymnophiona digresses from that in the forms thus far studied and to a slightly greater degree resembles that in the Birds, remains, therefore, to he noted.

(light stipple) and mesoderm (horizontal

The Gymnophionian Blastopore. — A surface view of the blastederm as gastrulation commences (Fig. 149, A), will reveal the fact that the posterior portion of the rim where involution is occurring has the shape of a wide crescent, whose ends or horns bend backward. As the process goes on, moreover, these horns continue to grow posteriorly, and presently turn toward one another until they meet (Fig. 149, B, C, D). It is furthermore to he noted that this movement has occurred relatively rapidly, whereas the epiholy of the anterior side of the hlastoderm, so rapid in the Fish, has scarcely started. The results of these processes compared with those in the Teleosts, as well as with those in forms with less yolk, may now be stated as follows:

If the entire blastodermal rim is still regarded as the lip of the blaste somatic splanchnic ; mesoderm and the ectoderm. The extent of the coelom at this stage is exag gerated in the diagram. ‘pore ( germ ring), it must be said that the movements ‘just noted have divided this lip into two portions.‘One of these is quite limited; 'i.e., it merely furnishes the boundary for the small area of yolk (yolk plug) enclosed by the fused horns of the crescent (Fig. 149, C ). The second portion of the original lip, on the other hand, bounds the entire remaining expanse of uncovered yolk. Moreover, since epiboly has been slight, this expanse comprises almost as much yolk surface as existed prior to the beginning of gastrulation. Such is the situation thus far indicated.

Fig. 147. A diagram to illustrate the method of gut formation and yolk coverage in the Fish. Note that the endoderm has folded in to form the gut without covering the yolk at all, i.e., there is no endoderm in the yolk—sac. The latter is covered by the periblast (not a permanent cell layer) and by the two layers of mesoderm

Upon the basis of subsequent development, however, it may be stated that the small area enclosed by the horns of the crescent is the only part which really corresponds to the blastopore in the forms previously studied. Hence, as would be expected, its ultimate closure gives rise to a line of tissue quite homologous with the typical primitive streak, the neurenteric canal arising at its anterior end and the anus at the other. From this it appears that in the Gymnophiona, the anterior and most of the lateral parts of the blastodermal rim take no part in forming the ventral and lateral lips of the region which must be homologized with a true blastopore, these lips being formed by the horns of the crescent. Instead, the outer (anterior and most of the lateral) portions of the rim are occupied merely with the gradual covering of the main body of the yolk, long after the true blastopore has been definitely delimited. Whether any convergence takes place before or during this delimitation has not been ascertained. Very possibly it does.

Fig. 148. Formation of the germ layers in Hypogeophis (a Gymnophionian). From Jenkinson (Vertebrate Embryology). After Brauer. A-—C. Sagittal sections of three successive stages. D. Transverse section through the blastopore and yolk plug ()r.p.l. s.c. Segmentation cavity into which in B and C the archenteron (arch.) opens. d.l. Dorsal lip. l.l. Lateral-lip. v.l. Ventral lip.

Fig. 149. Formation and closure of the blastopore in the Gymnophione. From Jenkinson (V erlcbrate Embryologyl. A—D. Surface views of the blastoderm of Hypogeophis. After Brauer. The lateral lips are seen to" meet behind and so form the ventral lip. y.p. Yolk plug. E. Embryo of Ichthyophis lying on the partially segmented yolk which is still uncovered by the blastoderm. After the brothers Sarasin.

It may now be noted that it is with respect to the relation of gastrulation proper and the belated enclosure of the yolk that the Gymnophiona come a step nearer to the condition in the Bird. In the latter also, as we shall see, gastrulation, so far as the embryo is concerned, is completed long before the mass of the yolk is covered by the epiboly of the blastodermal rim. However, this is as far as the resemblance goes. The Bird. it now appears, has no true blastopore related to the embryo itself, and the so-called primitive streak. if homologous with a blastopore, is formed in a different manner from any of the streaks so far described.

The Differentiation of Mesoderm, Notochord, and Definitive Endoderm

By means of the above processes of epiboly and involution, there is presently developed a telolecithal gastrula, whose lower or endodermal layer forms a roof for the former segmentation cavity (now the archanteron) in much the same way as in the Teleosts. In the present case, also, this layer soon gives rise to the mesoderm and notochord. The latter originates quite as in the Fish, but the formation of the mesoderm differs in the way previously noted as characteristic of other Urodeles. Thus in the Teleost it will be recalled that, though the development of the notochord involved all of the hypoblast in the median line of the embryo, the mesoderm on either side was merely split oil‘, leaving a layer of endoderm beneath it. In the Gymnophiona, on the other hand, the entire central portion of the archenteric roof which did not go to form the notochord becomes mesoderm (Fig. 150) . There is no delamination, and the result is that within the central area of the blastoderm, the enteric cavity for the time being is roofed only by mesoderm and notochord. In other words, in this case the central portion of the mesoderm, as well as the notochord, consumes in its formation all of the hypohlast beneath it. Presently, however, the encloderm in this central region is supplied by the ingrowth of lower layer cells from about the margin (Fig. 150). The uppermost layer as usual is now termed ectoderm and, as in the forms previously studied, all three layers are continuous with one another about the lips of the blastopore.

Fig. 150. Transverse sections of Hypogeophis showing the differentiation of the roof of the archenteron into notnchord (»n.ch.) and mesoderm and the formation of the gut (al.c.) by upgrowth of yolk-cells from the sides. From Jenkinson (Vertebrate Embryology). After Brauer.

As will presently appear the methods of mesoderm and notochord formation in the Teleosts and Gymnophiona are not particularly significant as regards an understanding of these processes in the Bird. Yet, because as usual, their occurrence somewhat overlaps gastrulation as strictly defined, an account of their character has been included for the sake of completeness.

References to Literature

CHAPTER VII

B auer, A., “Beitriige zur Entwickelungsgeschichte der Gymnophionen,” Zoal. ’ Jahrb.,X, 1897.

Brummett, A. R., “The relationships of the germ ring to the formation of the

tail bud in Fundulus as demonstrated by the carbon marking technique,” Jour. Exp. Zob'l., 1954-.

Hertwig, 0. (Editor), Handbuch der vergleichenden und experimentellen Entwicke lugslehre der Wirbeltiere, I, 1, 1, “Die Lehre von den Keimhl$a'.ttern,” Jena, 1903 (1906).

Hertwig, O. and 11., “Studies on the Germ Layers,” Jena Zeitschn, XIII—XVI (VI-IX), 1879-1883.

His, W., “ Untersuchungen iiber die Entwickelung von Knochenfischen, besonders iiher diejenige des Salmens,” Zeit. Anat. Entw., I, 1876.—“Untersuchungen iiber die Bildung des Knochenfischembryo,” Arch. Anat. u. Enzw., 1878.

Jenkinson, J. W., Vertebrate Embryology, Oxford and London, 1913.

Kopsch, F., Untersuc-Izungen. iiber Gastrulation und Embryobildung bei den Chordaten, “I. Die Morphologische Bedeutung des Keimhautrandes und die Embryobildung bei der Forelle,” Leipiz, 1904.

Korschelt und Heider, Lehrbuch der vergleichenden Entwickelungsgeschichte der wirbellosen Thiere, I, “ Experimentelle Entwickelungsgeschichte,” Jena, 1902. ———Lehrbuch, etc., III, “ Furchung und Keimblatterbildungf’ Jena, 1909-1910.

McEwen, R. S., “ The Early Development of Hemichromis bimaculata with Special

Reference to Factors Determining the Embryonic Axis,” Jour. Morph. and Physiol., XLIX, 1930.

Oppenheimer, J. M., “Processes of localization in developing Fundulus,” Jour. Exp. Zob'l., LXXIII, 1‘936.—“Potencies for differentiation in the teleostean germ ring," Jour. Exp. Zo5l., LXXIX, 1938.

Sumner, F. B., “Kupfiefs Vesicle and its Relation to Gastrulation and Concrescence,” Mem. N. Y. Acad. Sci., II, 1900.——“A Study of Early Fish Development: Experimental and Morphological,” Arch. Entw.-mech., XVII, 1903.

Wilson, H. V., The Embryology of the Sea Bass (Sermnus atrarius), (Bull. U. S. Fish Commission, IX), 1889.

• 1 Brummett ’54 has made a study of gastrulation in the fish, F unziulus, marking the blastodermal margin (germ ring) with carbon particles instead of stain, and concludes that, somewhat contrary to Oppenheimer and others, there is very little confluence or convergence in this form. Only the regions of the ring at, and quite near' (less than 90 degrees from) the incipient dorsal lip, are involved, and they form only the extreme posterior of the embryo and tail bud.

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