No. 3.] PODARKE OBSCURA VERRILL. 407 



The upper pole is indicated by the presence of the polar 

 globules, which lie between the cells, almost inside the seg- 

 mentation cavity (PI. XXXVI, Fig. 5). 



Eight to Sixteen Cells. — The i6-cell stage is reached by the 

 division of each cell of the 8 in a leiotropic direction, the divi- 

 sions all occurring at about the same time (PI. XXXVI, 

 Figs. 6 and 7). The division at the lower pole is equal and 

 leads to the formation of the second group of ectomeres. It is 

 of importance to note here that all of the cells of this second 

 group are equal in size, as compared with many other anne- 

 lids, in which a very large 2d, 

 "the first somatoblast," appears. 

 Inasmuch as the history of this 

 cell 2d in Podarke is similar to 

 that in other annelids, I shall 

 also call it the " first somato- 

 blast." At the upper pole the 

 division is more unequal, the 

 upper cell being the larger. (See 

 PI. XXXVI, Figs. 6 and 7.) 

 Since the eight cells were all 

 equal, it follows that now the 



largest cells are at the upper fig. i.- optical section of i6-ceii stage, show- 

 pole, and this relation holds until '"^ large cleavage cavity, pg, polar globules. 



the cross has been broken up into small cells (PI. XXXVIII, 

 Fig. 23 ; PI. XXXIX, Fig. 37). The smaller cells, ia2, iba, 

 IC2, id 2, divide twice more and form the primary prototroch 

 of the larva. I shall call them, after Mead, the primary trocho- 

 blasts. The segmentation cavity present as a lenticular space 

 in the 2-cell stage is continued through the later cleavages, 

 and is present as a good-sized cavity at this time. Its pro- 

 portions a little later than this are shown in Text-Fig. 2. 



Sixteeji to Thirty-tzvo Cells. — The 3 2-cell stage is reached by 

 the dexiotropic division of each of the sixteen cells, the transi- 

 tion falling into three well marked subdivisions. /. The division 

 of the macromeres to form the third group of ectomeres, which, 

 as we shall see, are not purely ectodermal in destiny, and the 

 division of the upper cells of the embryo giving rise to the 



