ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



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cells are ultimately tnvaginated and form endoderm (text figs. XXI to XXIV) and 

 arc 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 Cynthia. 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. 



a 716 a 7 'a 



B7.i a 7 ' 



XXIV 



Figs. XX1-XXIV. Sections of eggs of Cynthia partita; Figs. XXI and XXIV in the median plane. 

 Fig. XXIII a little to one side of the median plane at the posterior end, Fig. XXII in a transverse plane. 

 Fig. XXI represents a 64-cell stage, Fig. XXII a 64-76 cell stage. Fig.XXIII a 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; 

 yolk is also present in the inner ends of the ectoderm and mesoderm cells. The polar hodies shown in 

 Fig. XXIII in dotted outline do not lie in the plaueof the section drawn, but in that of the next section of 

 ilie series. It is probable that the neural plate and chorda cells of this figure (k~-* and A>3) have already 

 divided in a transverse plane {v. fig. 131), and that these cells should therefore be labelled A 8 *? and \ 

 a> in Fig. XXIV. In Fig. XXII the mesoderm cell B 6 - 2 (Fig. XVIII.i has divided into a mesenchyme 

 cell (B"-3) and a muscle cell IB'"*), the former containing little and the latter much of the yellow protoplasm. 



(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 

 yelkvw protoplasm of this egg collects at the Lower (vegetal) pole and then moves up 



