284 MEAD. [Vol. XIII. 



this division becomes the stomatoblast. What becomes of the 

 other product .■* In Unio (p. 24, Figs. 42-47) Lillie describes 

 two or three small cells as "budded off" from a" before the 

 latter sinks into the interior to form the larval mesoblast. In 

 other words, d''^ divides into a group of cells, one of which is 

 the actual larval mesoblast. What becomes of the others .-' 



There is no reason for believing that the product of the 

 division of a^''^, which in Nereis becomes a stomatoblast, corre- 

 sponds in origin to that which in Unio becomes the larval meso- 

 blast. But there is excellent reason for believing that it does 

 not, for it is the lower product of the first division which in 

 Nereis forms the stomatoblast, and the tipper product which in 

 Unio gives rise to the larval mesoblast. 



The transition to the 64-cell stage. — The rest of the cells of 

 the 32-cell stage have the same origin and position in the 

 various forms under consideration, but the examination of 

 their morphological relations will be facilitated by comparing 

 their products after the next cleavage, namely, at the 64-cell 

 stage. The transition from thirty-two to sixty-four cells is itself 

 of interest when one compares Nereis with more regular forms. 



In Lepidonottis, Amphitrite, Clymenella, and Chcetopterus all 

 the thirty-two cells divide in a left-oblique direction, according 

 to the regular rhythm, and, except in Chcztopterus, so nearly at 

 the same time that an actual 64-cell stage results. 



Wilson divides the cleavage of Nereis into a 38, a 42, and a 

 58-cell stage, but the fact that in this annelid the division of 

 the cells is not synchronous need not confuse us. In Lepi- 

 dojiotiis, Clymenella, Amphitrite, and Chcetopterus all thirty-two 

 cells divide in the left-oblique direction, and it would seem from 

 Wilson's figures, that the same thing may be said of Nereis (of 

 course excepting A^, B^, and C^, which do not divide at all). 

 Therefore Wilson's statement that the transition fro7n the 

 ''spiral'' (oblique) to the bilateral type of cleavage occurs at this 

 stage, and the consequent inferences, may be questioned. 



The 6^-cell stage. — We will begin our comparison with the 

 cells at the animal pole. The apical rosette, consisting of four 

 very small cells, arises in exactly the same way in Nereis, Spio, 



