98 ORGANIZATION AND CELL-LINEAGE ОҒ ASCIDIAN EGG. 
portion of the crescent gives rise to the muscle cells, the lighter or clearer portions 
to mesenchyme. Inasmuch as the protoplasm which enters into the muscle cells 
and mesenchyme is localized with such definiteness in the unsegmented egg it can 
scarcely be supposed that the substances which are to give rise to the neural plate 
and notochord are not also definitely localized though they may not be directly visible.’ 
If this presumption is correct the visibly different organ-forming substances are 
by no means the only ones present. 
(2) The striking effect of this cytoplasmic differentiation is heightened by the 
manner in which localization takes place. -The downrush of the peripheral layer 
of yellow protoplasm to meet the entering sperm, the subsequent movement of this 
protoplasm together with the sperm nucleus to the posterior pole and the formation 
there of the crescent, the migration of the clear protoplasm to the lower pole, thence 
to the posterior pole and then to the center of the egg.—these phenomena are so 
evident and they occur so rapidly that they strike the observer with amazement. 
(3) Finally the bilateral character of this localization is most notable. In all 
other recorded cases of cytoplasmic localization the various substances become 
arranged in zones around the chief axis of the egg and the symmetry is apparently 
radial ; here the early stages of localization are also of this sort, and the gray upper - 
pole, the clear middle zone and the yellow lower pole of the Cynthia egg immedi- 
ately after fertilization are not unlike the localizations in the eggs of. Myzostoma or 
Strong ylocentrotus, but in the ascidian this apparent radial symmetry gives place 
almost immediately to a marked bilateral symmetry which is brought about by the 
movement of the protoplasm from the lower hemisphere to the posterior pole and 
the formation there of the crescent. 
Certain fundamental resemblances which run through all these cases of cyto- 
plasmie localization are so striking that they scarcely need any emphasis here. The 
existence in the unsegmented egg of a peripheral layer of protoplasm which is 
clearly distinguishable from the remainder of the egg is a phenomenon of very wide 
occurrence. /n most of the cases just named this peripheral layer aggregates at 
one or both poles of the egg after fertilization, and in animals belonging to phyla 
as far apart as annelids, echinoderms, mollusks and chordates the substances at the 
upper pole give rise to ectoderm, those at the lower pole to mesoderm, while the 
endoderm arises from the region intermediate between these two. Although many 
differences appear in the later development of these animals they do not detract 
from the value of these fundamental resemblances which apparently afford a sound 
basis for a comparative morphology of ova. 
1 Since this was written I have been able to distinguish the chora-neural-plate substance as early 
as the 2-cell stage; it is the light gray protoplasm at the anterior border of the dorsal hemisphere 
(figs. 28, 32 et seq.) Photomicrographs of living egg of this stage will be published soon in which 
this substance is clearly shown. 
