454 K. M. PACE. 



twenty cells ; that is to say, there is an aboral series of 

 eiglit small cells which are disposed in two parallel rows and 

 belong to the fifth generation; below these a ring of eight 

 larger cells, four of which — A I, B ^, C |, and D I — are of 

 the fifth generation, and four others — A ^, B ^, C J, and 

 D^ — of the sixth generation; while the oral surface is 

 formed by four large central cells — A 5, B 5, C 5, and D I — 

 which arc also of the sixth generation, and which project 

 upwards into the segmentation cavity (PI. 24, ^g. 53). The 

 aboral series of cells contains, generally speaking, less yolk 

 than the lower series of cells, of which the before-mentioned 

 four large oral cells are particularly rich in yolk (PI. 24, 

 figs. 53, etc.). 



The 32-cell Stage. — The beginning of the next stage 

 is shown in PI. 24, figs. 54 a-h, and 55 a-h. ' The 

 small cells composing the two rows of the upper tier first 

 divide horizontally, giving rise to four rows each of four cells, 

 and arranged in two tiers (PI. 23, fig. 50 6, and PI. 24, figs. 

 54a-6, 55 a-h). This cleavage is followed almost imme- 

 diately by the division of the four cells — A 5, B ^, C 5, and 

 D I — which were the first cells originally budded off by the 

 large oral cells. These four cells divide (PI. 24, figs. 54 h, 

 and 55 h) by a vertical cleavage lying at an angle of 45° to 

 the primitive cleavage of the segmenting ovum. The embryo 

 (PI. 23, figs. 50 a.-c, and PI. 24, fig. 56) now, therefore, con- 

 sists of thirty-two cells, which arc all of the sixth generation, 

 and which are arranged in the following manner. The aboral 

 surface of the larva is composed of sixteen small cells, 

 disposed in two tiers, each tier consisting of two purallel 

 rows of four cells each. Below these sixteen aboral cells is a 

 ring of twelve larger cells, overlying and partially surrounding 

 the four large oral cells, which still occupy the lower surface 

 of the larva. The upper halves of these four oral cells, being 

 surrounded by the ring of twelve intermediate cells, are thus 

 enclosed by them within the segmentation cavity, and the 

 latter is almost entirely obliterated at this stage (PI. 24, 

 fig. 55 a). 



