ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 9 
could pass іп, after the manner recommended by Chabry. In most cases I have 
found that the presence of the egg envelopes does not seriously interfere with clear 
seeing, possibly owing to the fact that in the study of preparations I have used an 
immersion lens in which the depth of focus is relatively slight. In late stages, 
however, the test cells are sometimes confusing, and in the case of Crona I found 
that these, together with the other envelopes, could be removed by simply rolling 
the eggs under the cover glass. In Султа the envelopes may sometimes be 
removed in the same way, though not so easily as in Czoza. 
1. THE OVARIAN EGG. 
Much has been written on the egg envelopes and ovarian eggs of ascidians 
and I shall not here go over that ground in any detail. But in searching for the 
earliest differentiations of the egg substance it is necessary to go back to the 
ovarian egg, and in so doing I have found some structures the real significance of 
which has not hitherto been appreciated. 
1. Development of the Ovocyte. 
In a young ovocyte the cytoplasm stains uniformly and there is no trace of 
yolk or of test cells. Close around the nucleus is a granular mass which is deeply 
colored by plasma stains, the yolk nucleus or * yolk matrix " of Crampton (1899). 
As the egg grows, small spherules of yolk begin to appear in the vicinity of the 
yolk matrix, and this yolk gradually fills the central portion of the egg surrounding 
the nucleus, while the cytoplasm, which is free fram yolk, occupies a peripheral 
position. Some of the follicle cells which surround the egg at this stage then 
invade the egg, thus forming the “test cells" which are located chiefly in the 
peripheral layer of cytoplasm. Му observations as to the origin of these “test 
cells" agree with some of the most careful work, both ancient and modern, which 
has been done on this subject (Kowalevsky, 1866, 1871; Seeliger, 1882; Van 
Beneden and Julin, 1886; Morgan, 1890; Floderus, 1896; Bancroft, 1899). 
The earliest appearance of polarity is found in the location of the yolk matrix 
on one side of the nucleus and in a slight eccentricity of the latter. I consider it 
very probable that the yolk matrix is derived from the attraction sphere of the last 
ovogonie mitosis, and that the chief axis of the egg represents the cell axis which 
passes through the centrosome and nucleus, and which, as I have previously shown 
(Conklin, 1902), is preserved in every cell throughout the cleavage of the egg and 
probably also in all later cell divisions. If this be true, the polarity of the egg is a 
differentiation which is carried over from generation to generation, and as this chief 
axis of the egg is identical with the gastrular axis, and bears a constant relationship 
to the principal axes of the embryo and adult, it will be seen that at least one 
important differentiation of ап animal is predetermined (not predelineated) at all 
stages. Although this chief axis of the egg is usually recognizable at all stages by 
ә JOURN. А. М. 8. PHILA., VOL. XIII. 
