174 TORREY 



division of the first quartet (Text-Fig. i, E). These four cells 

 divide twice, as in Podarke, and form the primary protroch. 

 The first somatoblast is not indicated by any difference in size 

 at this stage. The 24-cell stage (Text-Fig. i, i^) is brought 

 about by the formation of the intergirdle cells and the third 

 quartet of micromeres by dexiotropic cleavages after which the 

 ^gg enters upon a resting period. The former divisions are 

 slightly earlier than the latter and mark the beginning of an ac- 

 celeration of divisions in the first quartet. Evidently here the 

 presence of yolk has nothing to do with the rapidity of cleavages, 

 as has been maintained by Kofoid ('95), but their rate is corre- 

 lated with the large size and early differentiation of the praetro- 

 chal region (Lillie, '95 ; Conklin, '98). As in Podarke, the 

 cells of the third quartet are slightly smaller than those of the 

 second. 



The 32-cell stage (Text-Fig. 2, A) is attained by the nearly 

 simultaneous dexiotropic and equal division of the four cells of 

 the second quartet and the four primary trochoblasts (Text- 

 Fig. 2, D). The arrangement of the blastomeres conforms now 

 exactly to what one would expect in the ideal ovum, as is also 

 the case in Podarke and Lcpidonotiis, namely : 



First quartet 1 6 cells. 



Second quartet 8 " 



Third quartet 4 " 



Macromeres 4 " 



32' «' 



The cells of the fourth quartet next arise leiotropically and 

 practically simultaneously (Text-Fig 2, A and E). There is 

 nothing in size (Text-Fig. 2, 6") or time of formation to dis- 

 tinguish the second somatoblast or J/ cell (4c/) from the others.^ 

 During these cleavages there has been a steady shifting to the 

 right of the upper part of the embryo on the lower (Text-Fig. 

 2, B), so that the polar furrows are now almost parallel instead 

 of at right angles (Text-Fig. 2, C\ Immediately after the 



1 In some forms the yJ/cell arises still earlier (in Crepidula at the 25-cell stage), 

 but its formation may also be greatly delayed as in Ischnochitoji (Heath, '99), 

 where it does not arise until the 72-cell stage, and then after the other three mem- 

 bers of the quartet have been budded off. 



