98 ORGANIZATION AND CELL-LINEAGE OF 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. 1 

 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 la}'er 

 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 Myzosloma or 

 Strongylocent rotus, 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- 

 plasmic localization are so striking that the}- scarcely need any emphasis hei'e. 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. In 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 chorda tes the substances at the 

 upper pole give rise to ectoderm, those at the lower pole to mesoderm, while the 

 endoderm arises frotn 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 heen 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. 



