326 PROCEEDINGS OF THE ACADEMY OF [July, 



small an amount of cytoplasm and so large a quantity of j'^olk. The 

 superficial layer of C3^toplasm is so thin and weak that at first thought 

 it would seem incapable of constricting and ultimately dividing this 

 great cell. But although the factors named seem inadequate to pro- 

 duce so large a result it must be borne in mind that the yolk is relatively 

 soft, almost fluid, and that a considerable ciuantity of cytoplasm is 

 distributed through the yolk at this stage; furthermore the rate of 

 division is extremely slow, many hours being necessary for the com- 

 pletion of each of the first two cleavages. These are the only equal 

 divisions of the entire yolk which I have seen. Although the four 

 macromeres give rise to many cells in the course of development they 

 still constitute the chief volume of the embryo as late as the veliger 

 stage (pi. XXVIII, fig. 31), thus showing that the yolk content of these 

 cells has remained practically as in the 4-cell stage. 



3. SEGREGATION OF THE GER.MINAL liAYERS. 



The third cleavage is very unequal and separates the first quartet 

 of micromeres from the macromeres in a dexiotropic direction (pL 

 XXIII, fig. 4); these cells come to lie in the furrows between the 

 macromeres, and their shape is consequently like the corresponding 

 cells in Crepidula. 



The fourth cleavage (fig. 4) separates the second quartet of micro- 

 meres from the macromeres in a liieotropic direction. In anticipation 

 of this cleavage the nuclei with the surrounding cytoplasm rotate 

 Iseotropically until they lie in the upper left-hand corner of each cell, 

 then the mitotic figures appear and the cleavage takes place precisely 

 as in Crepidula and other gasteropods. 



The first quartet cells then divide Iseotropically, giving off at their 

 peripheral borders the "turret cells," and a little later the macromeres 

 again divide in a dexiotropic direction, giving rise to the third cjuartet 

 (fig. 5). Simultaneously the cells of the second quartet divide 

 dexiotropically. These divisions are shown in figs. 5 and 6. As a 

 result of these divisions twenty micromeres are formed, of which eight 

 belong to the first quartet, eight to the second and four to the third 

 (fig. 6). The entire ectoderm comes from these twenty cells. The 

 micromeres are composed almost entirely of cytoplasm and are rela- 

 tively free from yolk; in the macromeres the cytoplasmic areas sur- 

 rounding the nuclei lie at the periphery of the cap of micromeres. 

 The visible quantity of the cytoplasm has increased very considerably 

 since the beginning of cleavage, as is readily seen by comparing figs. 

 1 and 6. This is probably due to the segregation of cytoplasm, origin- 



