ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 51 



Beneden and Juliti for Clavellina (1884, p. 13). Although this cleavage m;i \ be 

 subdivided into 20-cell, 22-cell and 24-cell stages, the duration of each of those 

 stages is very brief, and the fifth cleavage is completed in all the cells before the 



sixth appeal's. 



Castle ( IS'.IC. p. 229), in particular, has described the differences in the time of 

 division of the cells of the dorsal and ventral hemispheres, and has made it a prin- 

 cipal evidence in favor of his scheme of orientation. The fact that at this and at 

 the succeeding cleavage the cells of one hemisphere divide earlier than those of the 

 other has been accepted by him as proof that the earlier dividing hemisphere is 

 ventral and ectodermal, while the more slowly dividing one is dorsal and endo- 

 dermal, since, at the time of gastrulation, the number of cells of the ectodermal 

 hemisphere is greater than that of the endodermal. But neither the fifth nor 

 the sixth cleavage results in the formation of more cells in one hemisphere than 

 in the other, since all the cells of both hemispheres divide before the next cleavage 

 begins; at the close of the fifth cleavage there are sixteen cells in each hemisphere, 

 and at the close of the sixth cleavage thirty-two cells in each hemisphere. In the 

 seventh cleavage, as we shall see. the hemisphere in which divisions were slower at 

 the two preceding cleavages becomes the more rapidly dividing one, and thereafter 

 the number of cells is more numerous in this hemisphere than in the opposite one. 



In the anterior-dorsal cells the fifth cleavage spindles are parallel with the 

 median plane and are obliquely posterior-dorsal and anterior-ventral in direction 

 (fig. 117); four of the resulting daughter cells (A 6,2 , A 6 - 4 ) lie around the anterior 

 border of the egg just below the equator, while the other four (A 111 , A 6 - 3 ) form a row- 

 across the dorsal surface of the egg just in front of the second cleavage plane (fig. 

 117). The former are composed of yolk and protoplasm in about equal parts, and 

 give rise to chorda and neural plate cells; the latter are rich in yolk, but have little 

 protoplasm and give rise to endoderm. 



The four posterior-dorsal cells divide a little later than the anterior ones, and the 

 spindles lie approximately in a transverse direction (figs. 117, 189). The protoplasm 

 of these cells is chiefly crescent substance ; the small posterior cells (B 52 ) are almost 

 entirely composed of this substance, while the larger cells (B 3-1 ) are composed of this 

 substance and yolk in about equal proportions, the former occupying the outer half 

 of the cell and the latter the median half. These larger cells divide equally so as to 

 cut off all of this crescent substance and a small amount of yolk in the lateral 

 daughter cells and to leave hut little protoplasm and much yolk in the median ones 

 (figs. 37. 39). This division occurs at the 20-cell stage, and when it is completed all 

 of the mesodermal or crescent substance is finally and completely separated from 

 the endoderm, and, except for a small amount of yellow protoplasm which lies close 

 around the nuclei of many of the blastomeres, all the crescent substance is contained 

 in the four cells which form the posterior border of the dorsal hemisphere (figs. 39, 

 40). The small posterior cells divide a little later than these larger ones and 

 unequally, the median daughter cells being smaller than the lateral ones (figs. 41, 

 \'i. 119). Thus there come to he six mesodermal cells, three on each side of the 

 mid-line, during this cleavage. 



