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, et seq.), 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 (tigs. 113, 115, ISO, 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 the 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. 4(i, ct 

 seq.). The protoplasm of the anterior-dorsal cells is located at the anterior borders 

 of those cells (figs. 37. 113). 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 all the regions of 

 the gastrula and certain important organs of the later larval stage are here 

 actually marked out on the egg at the i6-cell stage. This is no ideal mapping out 

 of the egg into organ forming germ regions, but an actual localisation 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 positions 

 in the larva, and are composed of the same peculiar substances, as in the early 

 cleavage stages or even in the unsegmented egg. 



5. Fifth Cleavage; i6- 3 2 cells. (Figs. 3'.>-42, 116-11'.). 1S'.)-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 digs, llli-118. lS'.i. 

 I'M)). In Cynthia 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 (lii:. 118). In Cioua. 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 



