ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 51 
Beneden and Julin for Clavel/ina (1884, p. 18). Although this cleavage may be 
subdivided into 20-cell, 22-cell and 24-cell stages, the duration of each of these 
stages is very brief, and the fifth cleavage is completed in all the cells before the 
sixth appears. 
Castle (1896, 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 
(бо. 117); four of the resulting daughter cells (A*?, A**) lie around the anterior 
border of the egg just below the equator, while the other four (A*', A*?) form а 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?) are almost 
entirely composed of this substance, while the larger cells (В?!) 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 
eut off all of this crescent substance and a small amount of yolk in the lateral 
daughter cells and to leave but 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. 59, 
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, 
ES 42, 119). Thus there come to be six mesodermal cells, three on each side of the 
 mid-line, during this cleavage. 
