ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 21 
In Стопа the same type of protoplasmic movement occurs as in Cynthia, but 
with certain minor differences. Тһе peripheral layer is here decidedly thicker at 
the lower pole than elsewhere, even before the fertilization of the egg; the nuclear 
plasm or clear protoplasm is also at this stage distributed as a layer over the entire 
upper hemisphere of the egg (fig. 172). After the entrance of the spermatozoon 
the protoplasm of both these layers collects at the lower pole. Тһе nuclear plasm 
and peripheral protoplasm cannot easily be distinguished in living eggs of Crona, but 
in fixed and stained material the latter stains more deeply than the former (figs. 
172, 173). А crescent of peripheral protoplasm is formed here in the same way as 
in Cynthia (figs. 175, 176), and it occupies the same relative position (figs. 179— 
183). Though Castle did not observe the peripheral layer of protoplasm and its 
movement to the lower hemisphere it is evident that he recognized at least a part 
of the crescent. His figures 17 and 45-47 show the middle portion of the сгез- 
cent in the 2-8 cell stages, and he describes this as an area of finely granular pro- 
toplasm, which is clear in the living egg, and out of which the small posterior 
mesenchyme cells are formed. According to my observations these cells arise from 
a small part only of the middle portion of this crescent, while the greater part of 
the crescent gives rise to the muscle and mesenchyme cells of the tadpole. From 
his figures, as well as his descriptions, it is evident that he recognized only a small 
portion of the crescent, v7z., this median area of “clear protoplasm.” 
In many eggs of Czoza, if not in all, clear protoplasm, which is composed of 
large alveoles, surrounds the entering spermatozoon (fig. 175). Later, when the 
sperm nucleus moves to the posterior pole, this elear area moves with it, and in sec- 
tions in the median plane (figs. 175, 176) forms a elear triangular area in the middle 
of the deeply staining crescent. There is here shown a marked differentiation of the 
substance of the crescent which continues to be recognizable throughout most of the 
cleavage. І have not observed this clear median portion of the crescent in the 4-cell 
stage, but in the 8-cell stage and thereafter it is plainly visible as a deeply staining 
cap of protoplasm on each side of the mid-line. It corresponds in the main to the 
“clear protoplasm” described by Castle, which, as he discovered, marks the posi- 
tion of the sperm nucleus at the posterior pole and which ultimately gives rise to 
the “small posterior mesenchyme " cells (В?) at the posterior pole of the gastrula. 
This same clear protoplasm is present in the middle of the crescent in the Cyzthza 
egg, although it is here obscured by the surrounding yellow pigment; in the un- 
segmented egg it forms a layer of transparent protoplasm over the surface of the 
crescent, and in the cleavage stages of prepared eggs it is visible as two deeply stain- 
ing caps of protoplasm similar to those in the egg of Czona; it ultimately gives rise 
to the small posterior mesenchyme cells which are formed from the middle of the 
crescent and which are composed of clear protoplasm in which there is no yellow 
pigment (fig. 48, m’ch.). The substance of the crescent is therefore plainly differ- 
entiated from the first into these two substances, clear and yellow protoplasm, 
which remain distinct throughout the entire development. 
