90 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG 



characteristic of many, if not of all cells, and as it is present in all the cells of 

 the cleaving egg, where it is preserved from one cell generation to another (v. 

 Conklin, 1902), it may be considered to be a differentiation which is continuous 

 from generation to generation. But while the cell axis determines the egg axis 

 and this the gastrular axis, it is not necessary to suppose that in the early 

 ovarian history of the egg one pole is composed of ectodermal substance and the 

 other of endodermal. On the contrary, this is probably not the case. My obser- 

 vations on the living eggs of ascidians and snails leads to the view that it is not 

 the extrusion of the polar bodies at one pole which causes that pole to become 

 the ectodermal one, but rather that it is the movement of the germinal vesicle with 

 its contained clear protoplasm to o?ie pole, and the spreading of this pro/op/asm 

 at this pole, which is the determining factor. In short it is the localization of 

 ectodermal substance at the maturation pole zvhich causes that pole to give rise 

 to ectoderm. I shall return to this subject in the section on localization. 



Whether other axes of the egg are predetermined before cleavage is in most 

 instances unknown. In a lew cases all the axes of the future animal are marked 

 out before fertilization ; for example, among insects and cephalopods, as is well 

 known, it is possible to identify anterior and posterior, right and left, dorsal and 

 ventral axes of the egg while it is yet in the ovary. In most cases, however, the 

 only axis which is recognizable before fertilization is the chief axis of the egg. 

 This is true of the ascidians. but here there are certain evidences, which will be 

 presented in the next section, that the other axes are already established, though 

 not directly recognizable until after fertilization. 



B. Symmetry. 



Van Beneden and Neyt (1887) suggested that bilateral symmetry may be 

 characteristic of all cells of bilateral animals, and Lillie (1901) has expressed a 

 similar view regarding the eggs of such animals. This hypothesis, if true, would 

 materially simplify the problem of the earliest differentiations and localizations of 

 the egg, but it is supported by little direct evidence ; in fact, it is surprising that in 

 most bilateral animals bilaterality appears so late in development, In most annelids 

 and mollusks the egg and early cleavage stages are to all appearances radially symme- 

 trical, and in many cases bilateral symmetry first appears with the formation of 

 the mesentoblast cell, 4d. In echinoderms bilaterality is said to appear first in the 

 gastrula stage ; in Amphioxus during cleavage ; in ascidians it appears immediately 

 after fertilization and before the first cleavage ; while in cephalopod and insect 

 eggs it appears during the growth of the ovocyte in the ovary. Wilson has 

 repeatedly expressed the view that characteristic differentiations, such as bi- 

 lateral symmetry, arise at different periods of development in different cases, and it 

 cannot be denied that the ocular evidence is in favor of this view. On the other 

 hand, there are certain considerations which lead to the conclusion that bilateral 

 organization may be present in the developing egg or embryo long before it is directly 

 visible. For example, in Neritina there are two groups of granules in the proto- 



