JEXXIXGS: DEVELOPMENT OF ASPLaVNCHXA HERPJCKII. 99 



of the smaller cells of the egg. The condition shown in his Figure 31 

 he {'91, p. 62) considers to be an exception. Later, he states that the 

 polar cell becomes surrounded by small spherules, indicating that it is 

 degenerating and falling to pieces. As shown in ray Figures 51 and 

 52 (Plate 6), the vesicle d^-'^ produced by the division of d^-^ in 

 Asplanchna also becomes surrounded by large granules or spherules; 

 but these are not of the nature assumed by Zelinka; they are the result 

 of a concentration of granules in the ventral cell c?®'-^, traceable from 

 the eight-cell stage onward. In view of these facts, it seems possible 

 that a similar division actually takes place in Callidina, and that the 

 small cell lying between ai and bi in Zelinka's Figure 31 is the small 

 product of this cleavage, — the equivalent of c?^-- in Asplanchna. 



Tessin, in his study of Eosphora, also failed to follow the cleavage 

 in detail to the 32-cell stage. He ('86, p. 282) speaks of " fortgesetzte 

 Aequatorialtheilungen " of the cells in the three smaller quadrants ; his 

 figures show the three quadrants composed each of a single row of six 

 cells (Figs. 22 and 23). This corresponds to the condition in Asplanchna 

 at a time when the dorsal cells (a^-*-c^-*) have divided equatoi-ially, but 

 when the remainder of these three quadrants are as yet undivided. 

 Next, all the cells, except the large ventral cell d°-'^, ai-e said to divide 

 meridionally. It is probable, therefore, that the formation of the minute 

 cell by the division of d^-^ was overlooked, and that the cleavage is 

 essentially as in Asplanchna. The cells of quadrant B are said to 

 divide unequally at this cleavage, as is also the case in Asplanchna. 



The sixth and later cleavages of the ectoderm have not been studied 

 in detail by other observers, so that a comparison of my results with 

 observations on other forms is impossible. Diagrams of the sixth, 

 seventh, and eighth cleavages for Asplanchna are given on pages 41, 46, 

 48, and 53, that for the eighth cleavage being incomplete. 



The divisions of the entoderm cells have been followed somewhat further 

 by Tessin and Zelinka, so that for these a comparison may be made. 



Nothing comparable to the unequal fifth and sixth cleavages of the 

 entoderm cell (forming the small vesicles d^-^ and d''-^), shown in Fig- 

 ures 38 (Plate 5) and 49 (Plate 6), have been reported by other 

 observers. 



liater than these the cleavage of the entoderm in Eosphora (Tessin, 

 '86) takes place as in Asplanchna (Figs. 64, 65, and 76-83, Plates 8-10) 

 up to a stage comparable with that shown in Figures 77 and 78 (Plate 9), 

 except that no cleavage of the smaller dorsal cell d^-- corresponding 

 to the unequal division which I have shown in Figure 80 (Plate 10) wag 



