22 LILLIE. [Vol. X. 



/ have no doubt that there is a causal relation between the rudi- 

 mentary condition of the pre-velar region and the slow and 

 irregular character of the cleavages of the first generation of 

 ectomeres wJiich forms this region. 



d^ is generally the first of the apical pole cells to divide. 

 The products are ^' and d^"", which abuts against x^. <;' gener- 

 ally follows, and </'•' about the same time. Then comes c^-^ 

 and rt', followed by «'•' ; 3' and <^'' are the last of the ecto- 

 meres of the first generation to divide. After their division 

 a spindle appears in d''-''. Fig. 51 exhibits an unusually 

 regular arrangement of these sixteen apical cells. It can 

 easily be seen there that, while the first division of the four 

 central cells «', b^, c\ and d^ was leiotropic, the second division 

 is dexiotropic. 



We have already noticed one division of the three anterior 

 members of the second generation of ectomeres. This division 

 was obliquely equatorial in the cases of b^ and c^, and much 

 more nearly horizontal in the case of a^. In this latter case 

 the variation is correlated with the formation of the larval 

 mesoblast. We shall treat the divisions of d"" (the first somato- 

 blast) and ^^-^ (the larval mesoblast) separately, and so have 

 now to concern ourselves simply with b^\ b""-^, ^^S c^ ^ and a^\ 

 b^"" and c'^'' lie nearer the lower pole than <5^' and ^^-^ ; <^^' and 

 b^-^ are of approximately equal size ; the same is true of c^'^ and 

 c^'^. These four cells divide almost simultaneously, though ^^•' 

 leads the other three by a little. The plane is in each case 

 nearly horizontal (Figs. 41, 42, 43, 45, and 47) ; each cell 

 divides somewhat unequally: in the cases of <5^-^ and c^"" the 

 smaller product lies nearer the vegetative pole. The reverse 

 happens with b^"" and r^' ; b^'^'' comes to lie in the angle 

 enclosed by a^, A, B, and b^-. This group of cells has a very 

 characteristic appearance, as shown in the figures {eg.. Figs. 

 42 and 43). Fig. 43 is an anterior view of the Q.g%, and shows 

 very clearly the divisions of b^-^, b^-^, and c^'^. The spindle of 

 division of <:^'-^, which I have several times seen, is shown in 

 Figs. 45 and 47. As for a^\ a single division of this cell is 

 represented in Figs. 48 and 49, and the two resulting cells are 

 shown in Figs. 50 and 51. 



