OF THi: CERiNriNAL LAYER« TN Ci AlS'l IM )PODA. 2o 



and ")4), tluTC takes place the differentiation of the ento-mesomere 

 cell D. Tin's often liappens as early as the stage of the sixth 

 quartet, as is shown in Fig. 54. The details of this event are 

 qnite like those of Siphonaria, and therefore need no further 

 explanation. Even the entomere-cell />, itself thus formed, is 

 like that of the former species. It lies anteriorly and ventrally 

 to its mother-cell or mesomere jNI, and of conrse has the cross 

 fni'row in its original relation. 



Besides this notahle phenomenon there always ensues still 

 another i-emarkable change in the process of cleavage, namely 

 the formation of the bilateral cells similar to those described above 

 in Slphonaria. The division of these new dnughter-cells is 

 quite similar to what takes place in the species just cited. The 

 two posterior ectomere-cells of the fourth generation c-^ and d,; ad- 

 joining the right and the left sides of the mesomere-cell jM now 

 begin to divide quite independently of their two anterior com- 

 panions. As is well shown in Fig. 53 an apical view of the 

 animnl pole, the spindles in these two cells lie almost horizontally 

 and are directed, unlike all the preceding ones, toward the 

 median axis, that is, in a somewhat radial direction. The spindle 

 usually appears first in the right cells c-. (Fig. 55), and produces 

 a small daughter-cell re (Fig. 50) which resembles an ectomere- 

 cell of the seventh generation, especially f/2.,.1 in its general 

 appearance. The division of the left cell c/o soon follows, produc- 

 ing the cell Ic. Although the two bilateral cells Ic and re are 

 propagated from the cells of the same generation they behave in 

 a slightly difterent manner. The left l)ilateral cell Ic at th(^ 

 beginning of its detachment lies very close to, or even in contact 

 with, the entomere-cell D^ (Fig. 56). Subsequently, by the 

 cleavage of the mesomere-cell it is at last brought moi-e toward 



