CLEAVAGE IN ANNELIDS. 



103 



sixty-four-cell stage, about five hours after fertilization ; but, 

 from this time on, important differences are to be noticed. 

 Briefly stated, these differences may be said to be connected 

 with the more rapid development of the descendants of 2d and 

 ^^in AinpJiitrite than in Podarke. Figs. 5 and 6 show the rela- 

 tive development of these two sets of cells in early stages, and 

 Figs. 9 and 10 show the proportional size of the descendants 

 of these cells in a later stage. In the figure of AmpJiitritc the 

 mesoblast consists of three cells on a side, while in Podarke 

 there is only one, the third division of /j-d resulting in the sepa- 

 ration of two small cells, which enter into the formation of the 



Fig. g. 



Fig. 10. 



Figs. 9 and 10 — A m/>hitrite and Podarke, respectively, in a late stage of segmentation ; 



Bt., blastopore. 



wall of the archenteron. Similar cells have been described for 

 Nereis and Aricia by Wilson,^ and for Crepiditla by Conklin. 

 Note in the figures the relative development of 2d and its 

 descendants (indicated by the letter x), when compared with 

 one another and with corresponding cells of the other quadrants 

 (indicated by Fig. 2). Evidently in AmpJiitrite the descendants 

 of 2d form a large part of the subumbrella, and the difference 

 between the development of these cells here and in Podarke, a 

 difference which was noticeable in earlier stagers, has now become 

 much more pronounced. 



^ Wilson, (a) "Cell Lineage of Nereis," yi^^^r;?. of Morph., vol. vi; (b^ "Consid- 

 erations on Cell Lineage," etc., Aniials of the N. Y. Acadetny of Scietices, vol. xi, 

 No. I. 



