294 WILSON. [Vol. IX. 



idea. I could find no further stage in their development, and 

 so am unable to explain their nature, but their resemblance to 

 certain stages in spermatogenesis is obvious. 



3. Development of Gemmule into Swim.mlng Larva. 



After the gemmule reaches its full size (see figure of mature 

 gemmule, PI. XV, Fig. 18), it next undergoes a process in a 

 measure analogous to the segmentation of an egg. It breaks 

 up into masses, which are at first large, but which themselves 

 break up into smaller and smaller masses, and ultimately into 

 the individual cells, PI. XVI, Figs. 21, 22, 23. There is no 

 regularity at all in this splitting up of the gemmule. The 

 masses of cells into which the gemmule first splits are of all 

 shapes, and this continues to be true of the further division of 

 the separate masses. Almost from the very start a certain 

 number of individual cells separate themselves from the gem- 

 mule masses. This is shown in Fig. 21, the splitting up of 

 this gemmule having got well under way. It is here seen that 

 cells are set free both between the masses and on the surface 

 of the gemmule, but that there is a special tendency for the 

 superficial cells to become free at an early period. In this 

 gemmule the superficial cells already form a nearly complete 

 investment of the gemmule masses. It is evident from a study 

 of masses such as a and b in Fig. 21, from the surf ace of which 

 cells are being set free, that the outer cells act as independently 

 motile units. They change their shape and creep away from 

 the tightly compressed mass of cells. In the course of the 

 separation of the individual cells, their nuclei undergo a change. 

 The nucleus enlarges to such an extent that the membrane, 

 nucleoplasm and nucleolus all become easily distinguishable. 

 As the gemmule splits up it increases very considerably in 

 size, fluid being absorbed which fills the spaces between the 

 separating masses. 



During the gradual breaking up of the gemmule masses the 

 superficial cells come to form a connected layer. The forma- 

 tion of this layer was begun in the gemmule shown in Fig. 21, 

 and in Fig. 22 the layer is established icct.). The cells of 



