Paper - The Organization and Cell-Lineage of the Ascidian Egg 8

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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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I. Ascidian Embryology

1. The orientation of the ascidian egg and embryo adopted by Van Beneden and Julin is correct, that of Seeliger, Samassa and Castle is wrong (pp. 20-37).

2. The cell-lineage given by Castle is correct for the early stages; from the 48-cell stage on it is wrong (pp. 5 65 U ) .

3. The egg axis corresponds very nearly with the gastrular axis; during the closure of the blastopore this axis is shifted so that it is no longer dorso-ventral as in the early stages, but is antero-ventral and postero-dorsal in direction in the larva (pp. 73, 75-77).

4. The relation of the germinal layers to the cleavage planes is very different from the account given by Van Beneden and Julin and by Castle, and is more nearly in accord with that of Seeliger, Davidoff and Samassa. All cells above the equator (3d cleavage plane) are ectodermal and neural plate cells; all below are endodermal, mesodermal and neural plate (pp. 47-48).

5. The factors of gastrulation are (a) change of shape of the cells of the animal and vegetal hemispheres, (b) overgrowth of the marginal cells (pp. 64-65). Peculiarities of the gastrula are foreshadowed in the egg at a very early stage (pp. 45. 50. 59).

6. The muscle and mesenchyme cells arise from a common base, the mesodermal crescent, which surrounds the posterior side of the egg just below the equator (pp. 19-21); ultimately these cells surround the posterior margin of the blastopore (pp. 51-55); the most laterally situated of these cells become the large muscle cells of the tail of the tadpole, the portion of the crescent lying nearest the dorsal mid-line becomes mesenchyme (pp. 61-07). In the overgrowth of the dorsal lip of the blastopore the muscle cells become separated from the mesenchyme (pp. 69, 84-87).

7. The chorda arises at the anterior border of the blastopore from yolk-laden cells which resemble endoderm (pp. 53, 61, 62, 70-72, 83, 84).

8. The neural plate arises on the anterior side of the egg from cells of both hemispheres ; it extends from the margin of the anterior lip of the blastopore to a point about one-third of the way from the equator to the animal pole (pp. 52-54, 61-63, 66-68, 70, 72, 73. 82, 83).

9. The nervous and muscular systems do not arise from a common base as claimed by Castle; there is no nerve ring around the blastopore (pp. 61, 72, 73).

10. A comparison of the early development of ascidians with that of Amphioxus and amphibians shows that there is fundamental agreement among them in axial relations of egg and larva, in bilaterality of cleavage, in the method of closure of the blastopore and probably in the origin and position in the embryo of the neural plate, the chorda and the mesoderm (pp. 73-87).

II. Cytological Results

11. The maturation spindles are peculiar; they have no centrosomes nor asters ; they are formed entirely within the nuclear area from nuclear linin and chromosomes ; their fibres at first radiate in all directions, and finally they form a barrel-shaped spindle. The chromosomes separate without any possible influence from centrosomes or traction on the part of spindle fibres (pp. 15, 16).

12. In the first and second cleavages a small nuclear spindle like those present during maturation, lies between the two large asters, and in Ciojia it is quite distinct, from them. The separation of the daughter chromosomes takes place here as in the maturation divisions (pp. 40, 41).

13. The spermatozoon enters near the lower pole and rotates after entering the egg so that its centrosome is directed forward ; the centrosome is derived from the middle piece of the spermatozoon and can be followed without interruption until it divides, at right angles to the copulation path, and gives rise to the sperm amphiaster and finally to the cleavage centrosomes (pp. 22-24). A netrum is formed in the division of all centrosomes (p. 40).

III. Organization of the Egg

14. In the ovocyte of Cynthia parti/a there is a peripheral layer of yellow protoplasm, a central mass of gray yolk, and a large clear germinal vesicle, which is eccentric toward the animal pole. These same parts are jiresent in the eggs of other ascidians, but are differently colored (pp. 11, 12).

15. When the wall of the germinal vesicle dissolves at the beginning of the maturation divisions a large amount of clear protoplasm, containing dissolved oxyebromatin, is liberated into the cell body. This clear protoplasm is eccentric toward the animal pole and is distinct from the yolk and peripheral layer (pp. 13, 17).

lii. Immediately after the entrance of the spermatozoon the yellow and clear protoplasm How rapidly to the lower pole, where the yellow protoplasm collects around the point of entrance; the clear protoplasm lies at a deeper level. The yellow protoplasm then spreads out until it covers the surface of the lower hemisphere. This flowing of protoplasm to the point of entrance of the sperm is comparable with what takes place in many animals, though here much more extensive and rapid than elsewhere (pp. 1921, 77).

17. The withdrawal of protoplasm from the upper pole leaves the maturation spindles closely surrounded by yolk. The polar bodies are thus formed at the middle of a yolk-rich hemisphere, which is, however, the animal pole and not the vegetal pole as was claimed by Castle (pp. 19-21, 29, 30, 36, 37, 87-90).

18. The sperm nucleus moves from the point of entrance toward the equator in a path which is apparently predetermined. Tins path lies in the plane of the first cleavage and the point, just below the equator, at which the sperm nucleus stops in its upward movement, becomes the posterior pole of the embryo. The median plane and the posterior pole are probably not determined by the path of the spermatozoon, but by the structure of the egg. All the axes of the future animal are now clearly established, antero-posterior, right-left, dorso-ventral (pp. 22, 26, 90-93).

19. As the sperm nucleus moves to the posterior pole the clear and the yellow protoplasm move with it ; the latter collects into a yellow crescent with its middle at the posterior pole and its horns extending about halfway around the egg just below the equator. This position it retains throughout the whole development, giving rise to the muscle and mesenchyme cells mentioned in 6 (pp. 19-21, 97, 98).

20. After the sperm and egg nuclei have met at the posterior pole they move in toward the center of the egg and the clear protoplasm goes with them ; the outplace where the latter remains in contact with the surface is along the upper border of the crescent. At the close of the first cleavage the nuclei and clear protoplasm move into the upper hemisphere, and thereafter, throughout development, this hemisphere contains most of the clear protoplasm and gives rise to the ectoderm (pp. 20, 21, 42, 102).

21. The yolk which before maturation was central in position is shifted toward the animal pole when the protoplasm flows down to meet the spermatozoon ; when the sperm nucleus and surrounding protoplasm move to the posterior pole the yolk is moved down around the anterior side of the egg to the lower pole, and when the clear protoplasm moves into the upper hemisphere the yolk is largely collected in the lower hemisphere. This yolk rich area gives rise to the endoderm (pp. 20', 33-35, 12, 102).

22. At the close of the first cleavage the principal germ regions of the embryo are visible in their definite positions and proportions, viz. : the muscle-mesenchyme crescent and the ectodermal and endodermal areas. The chorda and neural plate areas are also visibly different from surrounding areas at this stage (pp. 42, 50, 95, 97, 98, 108).

23. In many eases the cleavage planes do not follow the lines of differentiation but cut across them. Although cleavage is, under normal conditions, constant in form, it is less constant and fundamental than the type of localization, and the two are relatively independent (pp. 103, 104).

24. The chief factor of localization is protoplasmic flowing; cell division is a factor of subordinate value (pp. 102-104).

25. Experiments which demonstrate the totipotence of blastomeres or regions of the egg prove nothing with regard to the presence or absence of differentiation in those parts. Some eggs with a high degreee of differentiation have at the same time great capacity for regulation, e.g., those of ascidians ; 1 others with no greater differentiation have little regulative capacity, e. g., ctenophores and mollusks. Therefore the potency of any part of an egg or embryo is no satisfactory measure of the degree of its differentiation (pp. 93-95).

26. The organization of the ovocyte is not the initial organization. The yellow protoplasm (mesoplasm) of the Cynthia egg is probably derived, at least in part, from sphere material (archoplasm) which arose from the nucleus at the last ovogonic division. / The yolk (endoplasm) is formed by the activity of the "yolk matrix" (Crampton) which also is probably sphere material. The clear protoplasm (ectoplasm) is derived from the germinal vesicle at the first maturation division. Thus many important regions of the egg come, at least in part, from the nucleus, and a method is therein suggested .of harmonizing the facts of cytoplasmic localization with the nuclear inheritance theory (pp. 99-101)./

27. There are several distinct types of germinal localization. /The annelidmollusk type does not approach that of chordates or echinoderms in the earliest stages of localization more closely than in the cleavage or gastrular stages. There is no convergence toward a common type in the earliest stages (p. 104-109).

28. Embryonic repetitions (recapitulations), as well as many other homologies, probably result from similarities of egg organization common to each type (p. 109).

29. "Precocious segregation" is not a satisfactory explanation of the origin of germinal organization (pp. 109, 110).

30. The evolution of animals must be accompanied by an evolution of the type of germinal organization ; modifications of this organization are probably the immediate causes of evolution. Transformations which would be impossible in adults are readily brought about by modifications in the organization of the egg {e.g., inverse symmetry). Perhaps profound mutations or even the origin of distinct types may be so explained (pp. 110, 111).

  • 1 See footnote p. 95.

Cite this page: Hill, M.A. (2020, February 27) Embryology Paper - The Organization and Cell-Lineage of the Ascidian Egg 8. Retrieved from

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