ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 



the daughter cells are similar in appearance. By this cleavage the cells of the 

 ventral hemisphere are increased to thirty-two, and when the small posterior cells 

 (B 6,8 , B 6-8 ) of the dorsal hemisphere have divided there are thirty-two cells in this 

 hemisphere also, or sixty-four in the entire embryo, all in the seventh generation 

 (figs. 130, 131). Tabulating these facts we find that there are at the close of the 

 sixth cleavage the following cells : 

 Ventral hemisphere 



26 ectoderm cells, protoplasmic. 



6 neural plate cells, protoplasmic. 

 Dorsal hemisphere 



Hi endoderm cells, yolk laden. 



4 chorda cells, yolk laden. 



4 neural plate cells, protoplasmic. 



4 mesenchyme cells, light yellow protoplasm. 



2 anterior mesenchyme cells, clear protoplasm. 



2 posterior mesenchyme cells, clear protoplasm. 



6 muscle cells, deep yellow protoplasm. 

 ti4 cells. 

 At the beginning of the sixth cleavage the cells of the ventral hemisphere are 

 narrow and columnar, while those of the dorsal hemisphere are broad and flat (text 

 figs. XI. XII, XIX, XX). This condition prevails up to the 44-cell stage when 

 the cells of the ventral hemisphere begin to divide. During their division the 

 ventral cells become shorter and broader, and at the same time the dorsal cells, 

 which have passed into a resting stage, grow more columnar and much smaller in 

 surface area, and before the close of this cleavage the cells at both poles are col- 

 umnar and of about the same height (text figs. XIII. XIV, XXI). This change 

 in the shape of the cells of the two hemispheres, which begins during the sixth 

 cleavage, is not completed until the seventh cleavage of the ventral cells (figs. 133. 

 134. 198-204). During this change of shape there is no difficulty in distinguish- 

 ing the two hemispheres, for the endoderm cells are filled with yolk ami the meso- 

 derm cells with yellow protoplasm or crescent substance, whereas the cells of the 

 ventral hemisphere are largely protoplasmic (text figs. XVII-XXIV). Moreover, 

 the polar bodies are often attached to the egg at its animal pole throughout the 

 whole of this period (cf. figs. 1U0-204). 



I have already discussed the views of Van Beneden and Julin, of Samassa and 

 of Castle relative to the shape of the cells of the two hemispheres. Although Van 

 Beneden and Julin showed by their figures that the dorsal cells of the 32-cell stage 

 of Clavellina are fiat and the ventral ones columnar, whereas the dorsal cells of the 

 44-cell stage are columnar and the ventral ones fiat, they did not observe nor 

 attempt to explain this change of shape. On the other hand, as we have seen, 

 Samassa and Castle denied that such a change of shape took place, and they there- 

 fore reversed Van Beneden and Julin's orientation of all stages before the 44-cell 

 stage. I have already given what seems to me satisfactory and sufficient evidence 

 in favor of the orientation of Van Beneden and Julin, and against that of Samassa 

 and Castle, and I need not repeat that evidence here. 



