10 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
a slight eccentricity of the nucleus, it is often difficult to observe it after the dis- 
appearance of the yolk matrix. No other axial differentiations of the egg are 
recognizable until after the fertilization. 
2. Test Cells and Chorzon. 
In the fully formed ovarian eggs the test cells lie imbedded in a peripheral 
layer of clear protoplasm ; this layer stains intensely with plasma stains, and in the 
living eggs of Cynthza contains yellow pigment granules. In Cynthza the test cells 
are distributed singly and pretty uniformly in this peripheral layer (fig. 61), and 
the same is true of Czoza at an early stage in the formation of the ovocyte (figs. 168, 
169), but in the fully formed ovarian egg of Czoza the test cells are found in little 
masses or “nests” of from three to six or eight cells each (fig. 170). А similar 
grouping of the test cells has been described by Morgan (1890) in an unidentified 
species of CZave//zna. These cells are much smaller and more numerous than the 
test cells of Cynthza, and are evidently formed by division of the original test cells. 
The test cells of Cynthza become quite large and contain yolk spherules, though 
they do not stain as densely as the yolk of the egg; іп Crona the test cells are very 
much smaller and do not contain these spherules. About the time that the ovarian 
eggs escape from the ovary the test cells are extruded from this peripheral layer of 
protoplasm, and the outlines of the egg, which up to this time have been irregular, 
become more nearly spherical. It is probable that the expulsion of the test cells 
and the assumption of the regular spherical form by the egg have a common cause 
in the increase оҒ surface tension at this time. 
At the time of the extrusion of the test cells I have observed in the ovarian 
eggs of Crona a faintly-staining, homogeneous layer which lies inside the outer 
follicle cells and outside of the egg. Тһе test cells lie on the inner border of this 
homogeneous layer; from its general appearance it is highly probable that the sub- 
stance of which it is composed is extruded from the egg along with the test cells. 
This homogeneous material does not long persist as such but soon disappears and 
probably goes to form the chorion. At this time the egg undergoes considerable 
shrinkage in size, a distinct perivitelline space being formed, and the egg becoming 
regularly spherical (cf figs. 171 and 172). It is evident that this is due to the. 
escape of fluid from the egg, probably the homogeneous substance described above. 
In this connection a word or two as to the significance of the test cells may be 
permissible. The fact that in Cyz/Zza they contain yolk and grow to a considera- 
ble size, and that spermatozoa not infrequently enter them (figs. 80 and 85 sn.), may 
be taken as evidence that these cells are rudimentary eggs; a view which is held by 
Floderus (1896), Bancroft (1899) and others. 
3. Structure of Fully Formed Ovocyte. 
When first laid the living eggs of Cynthia are, exclusive of the egg envelopes, 
about 150 „ in diameter; those of Crona are about the same size, but in Molgula 
they are much smaller, being about 100 „ in diameter. The very large germinal 
