a аш. d 
ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 35 
cells are ultimately invaginated and form endoderm (text figs. XXI to XXI V) and 
are therefore dorsal in position, while the protoplasmic cells at the opposite pole. 
form ectoderm and are ventral in position. The cells which form the posterior 
boundary of the yolk-rich hemisphere contain the small spherules, already described, 
which are characteristic of the yellow protoplasm of Cynthza. The distribution of 
the yolk shows conclusively, therefore, that the cells of the animal or ventral hemi- 
sphere contain most of the clear protoplasm and give rise to the ectoderm, while 
the cells of the vegetal or dorsal hemisphere contain most of the yolk and yellow 
protoplasm and give rise to the endoderm and mesoderm. 
вв. XX1-XXIV.—Sections of eggs of Cynthia partita; Fi igs. XXI and XXIV in the median plane, 
Fig. Бі a little to one side of the median plane at the е posteri jor end, Fig. ХХП in a transverse plane. 
Fig. X XI represents a 64-cell stage, Fig. X XII a 64-76 cell stage, Fig. Хх а 76-110-cell stage, and Fig. 
XXIV a 110-cell stage. The clear protoplasm, the yellow protoplasm and the yolk are represented as in 
the preceding figures. The clear protoplasm is localized chiefly in the ectoderm and neural plate cells, 
the yellow protoplasm in the crescent cells (mesoderm) and the yolk in the endoderm and chorda cells; 
a (Br ss an ; Ms се ib de ones coctus аны кі М [=н сз er M Gan potions MEN, 
(2) This orientation is further confirmed by a study of the yellow crescent of 
the Cynthia egg and of the cells which develop from it. As has been shown, the 
yellow protoplasm ОҒ this egg collects at the lower (vegetal) pole and then moves ир 
