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 опе, but rather that 27 zs the movement of the germinal vesicle with 
tts contained clear protoplasm to one pole, and the spreading of this protoplasm 
at this pole, which 15 the determining factor. In short it zs the localization of 
ectodermal substance at the maturation pole which 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 few 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 itis 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 ав 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. Оп 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 /Verztzua there are two groups of granules in the proto- 
