Paper - The Organization and Cell-Lineage of the Ascidian Egg 5

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Conklin EG. The Organization and Cell-Lineage of the Ascidian Egg (1905) J. Acad., Nat. Sci. Phila. 13, 1.

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Conklin 1905 TOC: I. The Ovarian Egg | II. Maturation and Fertilization | III. Orientation of Egg and Embryo | IV. Cell-Lineage | V. Later Development | VI. Comparisons with A.mphioxus and Amphibia | VII. The Organization of the Egg | Summary | Literature Cited | Explanation of Figures

V. Later Development

My observations on the later stages of development agree in the main with those of previous workers and particularly with those of Castle, who made a thorough study of these stages by means of serial sections. It is true of these stages, however, as it is of the cleavage stages, that many topographical relations can be made out more satisfactorily by a study of entire preparations. I have therefore devoted especial attention to such preparations, and my observation, both on living and on stained material, are embodied in plates V and X.

1. Closure of Blastopore

During the closure of the blastopore the embryo changes shape and at the same time the egg axis is shifted. This stage is therefore an important one in the orientation of the later stages. The gastrula is at first disk-shaped (fig. 134), it then becomes saucer-shaped (fig. 136) and then cup-shaped (figs. 144, 145). During this change as the embryo increases in depth it decreases in its other dimensions so that it becomes more nearly spherical (fig. 145). The closure of the blastopore takes place more rapidly from the anterior than from the posterior side ; in fact after the general drawing together of the margins of the saucershaped gastrula the posterior lip remains nearly stationary until the last stage in the closure of the blastopore.

Soon after the 218-cell stage the gastrula becomes elongated and egg-shaped, the posterior end being somewhat narrower than the anterior. The anterior lip of the blastopore continues to grow 'posteriorly while the lateral lips draw nearer together; thus the blastopore becomes T-shaped (fig. 148), and finally, by the further growth of the anterior lip, the anterior part of the blastopore, represented by the bar of the T, is covered and the blastopore is reduced to a longitudinal groove between the lateral lips (figs. 152, 153). In the growth of the lateral lips they come to lie at a higher level than the anterior lip. and consequently as the latter continues to. grow posteriorly, the former are tilted up at their anterior ends until they become vertical in position. These lateral lips are at first formed only of the muscle cells, but later the ectoderm cells completely overgrow them.

In this process the ectoderm does not, for some time, close up the notch at the posterior end of the blastopore (fig. 52); this is one of the last steps in the process of closure.

The overgrowth of the anterior lip continues until it has covered about threequarters of the dorsal face; meanwhile the animal pole is shifted nearer to the point of greatest curvature at the anterior end. and the blastopore is transported from the dorsal side toward the posterior end. In this process the rows of muscle cells which at an earlier stage stretched from the posterior pole to the second cleavage plane, and were antero-posterior in direction, are tilted up at their anterior ends and pushed backwards until they lie at the hinder end of the embryo and run in a dorso-ventral direction (figs. 52, 53, 50, 157). This complete change in the direction of the rows of muscle cells I found most perplexing and difficult to understand. In early stages the crescent, and the mesoderm cells which form from it. lies just below the equator of the egg, and in the anteroposterior plane; in these later stages the muscle cells are transverse to the anterior-posterior axis. A detailed study of intermediate stages shows how this change is brought about. After the closure of the anterior part of the blastopore, corresponding to the bar of the T. the anterior lip does not overgrow the blastopore groove (stem of the T) and its lateral walls, which are composed of the muscle cells; on the other hand, these lateral walls lie at a higher level than the anterior lip, and the continued growth of this lip pushes the muscle cells and the groove before it. As the posterior lip remains stationary during this process it happens that the entire dorsal portion of the posterior quadrants is tilted up in front and pushed backward until it forms the posterior end of the embryo, the posterior lip becoming vental and the anterior lip dorsal in position. Thus the blastopore groove, which lay on the dorsal side posterior to the middle, comes to lie at the* posterior end of the embryo and the walls of the groove, containing the muscle cells, come to be terminal in position and vertical in direction (figs. 50-53). The mesenchyme cells, as well as the caudal endoderm. lie at so low a level that they are not disturbed by the overgrowth of the anterior lip, consequently the rows of these cells still preserve an antero-posterior direction (fig. 157). Thus the mesenchyme and muscle cells, which in earlier stages lay side by side, come to be separated anteriorly, and only remain in contact with one another at the hinder end of the strand of caudal endoderm cells; the mesenchyme cells in this region are derived from the median part of the crescent and they ultimately become separated from the remaining portion of the mesenchyme which comes to lie in the trunk (figs. 161-167).

These general changes in the shape of the embryo at this stage are accompanied by divisions of many of the cells, some of which we may now consider. In all the ectoderm cells the ninth cleavage is nearly synchronous ; in the posterior quadrants the spindles are approximately antero-posterior in direction, and the same is true for the two hindmost rows of the anterior quadrants, but in most of the other cells of the anterior quadrants the spindles are transverse, thus it happens that the animal pole is shifted forward (fig. 149). As compared with the eighth cleavage there is therefore a regular alternation in the direction of division in most of the cells. 1 have not observed the tenth cleavage of these cells, but it seems probable that the direction of division is, in many of the cells, the same as at the ninth cleavage, if one may judge by the longitudinal rows of cells as well as by the number of rows which are present in the posterior quadrants (figs. 155, 160). The animal pole is, therefore, shifted still further forward during this cleavage. The posterolateral ectoderm cells slowly overgrow the muscle cells, but for a long time they do not overgrow the median posterior mesenchyme cells, and there is therefore left the deep notch in the blastopore at the hinder end of the embryo which has already been described.

The neural plate in the stage shown in figure 148 consists of six transverse rows of cells, only four of which show in the figure. The two posterior rows are derived from the dorsal hemisphere and consists of eight cells each; the four anterior rows consist of six cells each, anil are derived from the ventral hemisphere. In subsequent divisions of the posterior rows of this plate the spindles are anteroposterior in direction, thus adding to the number of rows but not to the number of cells in each row; for example, in figure 152 there are eight rows of cells, but apparently only six cells in each row. 1

The endoderm cells of the anterior quadrants divide chiefly in a transverse plane; those of the posterior quadrants in an antero-posterior plane (figs, 150, 151, 156). This fact, taken in conjunction with the direction of division in the ectoderm cells, contributes to the lengthening of the posterior part of the embryo and to the widening of the anterior part, and consequently to the shifting of the animal pole further toward the anterior end.

Of the two rows of chorda cells established at the eighth cleavage one has come to lie posterior to the other, and both bend so as to become horse-shoe-shaped (fig. 153). Later these cells divide again (fig 157) and, pushing backward with the anterior lip. carry the muscle cells before them, as already described.

In the 218-cell stage there were twenty mesenchyme cells: in the next stage shown (fig. 150) these are increased by one or two divisions so that there are twentytwo or twenty-four cells. As in the preceding stage, they still lie on the ventral side next to the ectoderm and along the posterior border of the gastral endoderm. In figure 150 only one row of mesenchyme cells is found lateral to the caudal endoderm ; in figures 154 and 15G there are two such rows. In all these figures there are three pairs of mesenchyme cells at the hinder end of the caudal endoderm ; the most posterior of these is the small posterior mesenchyme cells (B 76 ), the others are B 89 and B 810 . All of these cells are protoplasmic, stain deeply and art' strikingly different in appearance from the endoderm cells.

The muscle cells, which in the 218-cell stage consisted of six pairs of cells, are shown in figures 51 and 153 increased to eight pairs which are arranged in two rows on each side of the blastopore groove. By the continued growth of the anterior lip these rows are tilted up into a dorso-ventral direction. An optical section of the caudal region at this static shows four muscle cells on each side, one above another (fig. 158) ; a lateral view shows three rows of muscle cells with four or five cells in each row (figs. 56, 157). I have not observed the exact manner in which this change from two rows to three takes place, but it is evident that it must be associated with the division of the cells of the original two rows. In figure 51 there are eight muscle cells on each side; in figure 55. thirteen ; in figure 56, fourteen; in figure 157. fifteen ; in figure 165, eighteen; and in figure 59, twenty; therefore each of the cells shown in figure 51 must have divided once and some of them twice during the period represented by these figures.

  • 1 In this figure it is possible that a single row of cells on each side of the stippled area should be reckoned as part of the neural plate.

2. Development of Larva

(Figs. 57-60, 160-167).

After the growth of the anterior lip has carried the notochord to a position approximately corresponding to that of the blastopore groove in figure 153 and has shifted the rows of muscle cells into a nearly vertical direction (figs. 157. 158), these rows of muscle cells again come to be antero-posterior in direction (figs. 58, 59, 161-165). This change takes place rather suddenly and I have not observed all the steps in the process. It seems probable, however, that it is due to two factors; (a) the depression of the dorsal ends of the muscle rows to a position alongside of the notochord, and (b) the outgrowth of the tail of the larva from the region of the ventral ends of the muscle rows. This outgrowth, which is associated with the lengthening of the ventral side of the embryo, carries the caudal mesenchyme cells and the ventral ends of the muscle rows backward into the tail and thus the rows of muscle cells again assume an anteroposterior direction (figs. 58, 59, 161-167). Usually six cells are seen in each row and in addition there are two or more cells at the hinder ends of these rows which do not fall specifically into any one of them. In livingembryos the muscle and mesenchyme cells retain their yellow color and the individual muscle cells may be plainly seen ; all the figures shown in plate V represent camera drawings of living embryos and in all of them the yellow cells were distinctly visible as drawn. 1 When seen from the caudal pole (fig. 60), the three rows of muscle cells are seen to be only one-layered and the cells of one side are connected with those of the other by a group of small yellow cells (the caudal mesenchyme), "which lie ventral to the notochord at its hinder end.

In stained preparations of young tadpoles these caudal mesenchyme cells can be seen to consist of two or three pairs of cells at the posterior end of the caudal endoderm and ventral to the notochord (figs. 101-165). The other mesenchyme cells, which in a former stage (figs. 156, 157) were continuous with this caudal group, are now separated from it by the whole length of the muscle rows. These mesenchyme cells at the anterior ends of the muscle rows are found in later stages cells ; in figures 161-165 they. consist of eight or ten cells on each side. Whether there may be a few scattered mesenchyme cells between the caudal and the trunk groups and ventral to the muscle rows must still be left an open question, but there can be no doubt that most of the mesenchyme cells are located in these two groups. The separation of the caudal from the trunk mesenchyme must have been accomplished in part by the same means which brought the muscle rows from a vertical to a horizontal position, viz., by the outgrowth of the tail. In addition there seems to have been an actual forward movement of the trunk mesenchjme, as is indicated by a comparison of such figures as 156 and 161. This is probably part of the general forward shifting of the animal pole. In later stages when the tail is bent toward the ventral side, the trunk mesenchyme is found ventral to the anterior ends of the muscle rows (figs. 59, 166, 167). In these later stages the mesenchyme cells are frequently found dividing; they are smaller and more numerous than the muscle cells and are more than one cell-layer thick.

  • Since this paper was sent to press Misses I'd. it and Strobell have prepared for me a series of

more than thirty photomicrographs of the living eggs and embryos of Cynthia. These photographs show in the most striking manner the yellow protoplasm and the cells which arise from it ; even in the tadpole stage these individual cells are plainly recognizable in the photographs.

in the trunk of the larva, and they may therefore be known as trunk mesenchyme

In the formation of the larva the ventral cord of endoderm increases greatly in length, being composed in very young tadpoles (fig. 161) of six or seven pairs of cells. These cells form a double row between the muscle cells of each side and ventral to the notochord. In front of the caudal endoderm and notochord lies the gastral endoderm consisting of yolk cells which form a single but rather irregular layer around a small central cavity, the enteron (figs. 161, 162, 164-166).

In young larva 1 the chorda cells are wedge-shaped and form two or more rows of cells which interdigitate, as has been described by previous writers. In the latest stage which I have studied (fig. 167) these cells interdigitate to such an extent that the}- form a single row of disk-shaped or slightly wedge-shaped cells. I have not followed in detail the method by which the two arcs of chorda cells shown in figure 153 are transformed into the double row shown in figure 162, but I see no reason to question the account given of this by Van Beneden and Julin and also by Castle.

The neural plate grows backward with the notochord nearly to the posterior end of the embryo. I can find no evidence in favor of the view that any portion of the nervous system is derived from cells which bound the blastopore groove posterior to the neural plate (figs. 152, 153), nor is there any evidence for the existence of a nerve ring surrounding the blastopore. Since the neural plate, six or eight cells wide at its hinder end, is carried back with the chorda nearly to the hinder end of the embryo where the last trace of the blastopore is found (fig. 53), and since no portion of the nerve cord is found posterior to the blastopore and notochord (figs. 163, et seq.), it seems most probable that the hinder portion of the nerve cord, as well as all the rest of it, is derived from the neural plate and not from the lateral lips of the blastopore groove. That the muscle cells do not give rise to the posterior part of the nerve cord, as claimed by Castle, is made probable by the fact that this portion of the nerve cord is not yellow, as are the muscle cells: I cannot therefore accept without further evidence Castle's statement that the posterior portion of the nerve cord is formed from the muscle cells (his "neuromuscular" cells). Furthermore, I am unable to find satisfactory evidence that the ectoderm which covers the muscle cells and closes the blastopore notch behind contributes to the formation of the nerve cord. Therefore, it is probable that the entire central nervous system conies from the neural plate, which is a portion of the anterior lip of the blastopore.

After having overgrown the muscle cells and closed up the posterior notch of the blastopore the ectoderm forms a pair of Y-shaped folds (figs. 52-54), the apex of the V lying just behind the blastopore and the limbs diverging anteriorly and laterally. By the forward extension of these folds the neural plate is rolled up into a tube which is covered with a layer of ectoderm, in the manner characteristic of vertebrates. These folds are at first V-shaped, but after they have extended around the anterior end of the nerve plate the}' inclose an oval area which is pointed behind (fig. 55). The folds close from behind forward and ultimately convert the entire neural plate into a tube, which retains a lumen in its anterior portion (the sense vesicle) and an opening to the exterior (the neuropore). but which contains no lumen back of the anterior end of the notochord (figs. 16G, 167). That portion of the nervous system dorsal to the notochord and which contains no lumen is derived from those neural plate cells which belong to the dorsal hemisphere and which in origin ivere intimately associated with the chorda cells ; the anterior half of the enlarged portion of the tierve tube lying in front of the notochord (sense vesicle) is derived from those cells of the neural plate which belong to the ventral hemisphere. As tiearly as I can determine the anterior end of the neural plate lies about jo above the original equator oj the egg and 6o below the anitnal pole. The cephalic pole of the larva lies ventral to the anterior end of the neural plate but dorsal to the animal pole ; therefore, the antero-posterior axis coincides neither with the egg axis nor with the equatorial plane but lies mid-way between the two. The egg axis is therefore not dorso-ventral in the larva but is, strictly speaking, postero-dorsal and antero-ventral in direction. Inasmuch as the forward shifting of the animal pole by which this position of the axes is brought about occurs at a late period in the development, and also for the sake of simplicity of expression I have, in accord with all my predecessors, described the egg axis as dorso-ventral in direction in all the early stages.

Conklin 1905 TOC: I. The Ovarian Egg | II. Maturation and Fertilization | III. Orientation of Egg and Embryo | IV. Cell-Lineage | V. Later Development | VI. Comparisons with A.mphioxus and Amphibia | VII. The Organization of the Egg | Summary | Literature Cited | Explanation of Figures

Conklin EG. The Organization and Cell-Lineage of the Ascidian Egg (1905) J. Acad., Nat. Sci. Phila. 13, 1.

Cite this page: Hill, M.A. (2020, February 29) Embryology Paper - The Organization and Cell-Lineage of the Ascidian Egg 5. Retrieved from

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