50 ORGANIZATION AND CELL-LINEAGE OF ASCIDIAN EGG. 
figure 37, and the formation of the small posterior cells shows most beautifully that 
the cleavage planes do not necessarily follow the lines of demarcation between the 
yellow protoplasm and the yolk; for in this case they cut across those lines so that 
the small posterior cells contain a wedge of yolk in addition to the yellow proto- 
plasm (figs. 37, 113, 117). This yolk is later obscured by being covered by the 
yellow protoplasm (fig. 39, e¢ seg.), but when the posterior cells are first formed it 
is quite distinct. These small posterior cells contain not only yellow protoplasm 
and yolk, but also those caps of clear superficial protoplasm which later go into the 
small posterior mesenchyme cells. These cannot be seen in the living egg, but are 
very evident in stained preparations (figs. 118, 115, 186, 187). 
The localization of yolk and protoplasm at the vegetal pole is now practically 
the same as at the beginning of gastrulation, and it is clearly indicative of the loca- 
tion of definitive organs. The relative positions of the yolk and yellow protoplasm 
are the same in ihe. 16-cell stage shown in figure 37, as in the early gastrula stage 
shown in figure 46. The area of yolk, free from protoplasm, which surrounds the 
vegetal pole (figs. 37, 111, 113), gives rise to the endoderm of the gastrula, the 
tongue of yolk which runs back between the arms of the crescent (figs. 37, 113) 
gives rise to the caudal endoderm cord of the larva, while the greater breadth of 
the yolk in front of the second cleavage plane (fig. 37, 113) is indicative of the 
great transverse extent of the endoderm of this region in later stages (fig. 46, e/ 
seg.). The protoplasm of the anterior-dorsal cells is located at the anterior borders 
of those cells (figs. 57, 115), and in this region the notochord and neural plate cells 
later arise. In all these respects the localization of these substances is of direct 
prospective significance ; in fact one may go further and say that а// the regions of 
the gastrula and certain tmportant organs of the later larval stage are here 
actually marked out on the egg at the 16-cell stage. This 15 no Тағай mapping out 
of the egg into organ forming germ regions, but an actual localization of strikingly 
different substances which need only to be followed through the development to 
prove that they give rise to definite organs which occupy the same relative posttions 
zn the larva, and are composed of the same peculiar substances, as tn the early 
cleavage stages or even in the unsegmented egg. 
5. Fifth Cleavage ; 16-32 cells. (Figs. 39-42, 116-119, 189-195), 
The fifth cleavage does not occur simultaneously in all the cells of the fifth 
generation, but divisions appear in the cells of the vegetal or dorsal hemisphere 
before they do in those of the animal or ventral hemisphere (figs. 116-118, 189, 
190). In Cynthza the anterior-dorsal cells divide a little earlier than the posterior- 
dorsal ones (fig. 117), and the anterior-ventral cells a little in advance of the pos- 
terior-ventral ones (fig. 118). Іп Czona, also, the cells of the dorsal hemisphere 
divide before those of the vental, but there is practically no difference in the time 
of division of the anterior and posterior cells of this hemisphere. Neither at this 
stage nor at any preceding or succeeding one are the cleavages more rapid or the 
cells more numerous at the posterior than at the anterior pole, as claimed by Van 
