90 MCMURRICH. [Vol. XL 



and the principal difference in the two forms seems to be the 

 smaller number of cells in Ascllus in the circle surrounding 

 the products of D. 



The next, or 32-celled stage, I have not, unfortunately, suc- 

 ceeded in obtaining, but from preparations of the 64-celled 

 stage I have been able to determine what the divisions may- 

 have been. In Fig. 32 is a view of the 64-celled stage in 

 which the cells which arise by the division of each of the cells 

 of the 32-celled stage are bracketed. Disregarding all other 

 cells than D and X of Fig. 31, each of these divides into two, 

 so there are four cells which correspond cytogenetically to the 

 four vitellophag cells of the 32-celled stage of Jaera. At the 

 next division these divide into the eight cells represented in 

 Fig. 32, as D^ E^ D^ D"^ X^'^, which present a well-marked 

 differentiation, the cells indicated by D being much smaller 

 than those which result from X. It will be noticed the X and 

 D groups of cells no longer are situated at the extremity of 

 the oval Q.gg, but occupy almost the middle of one of its faces, 

 that namely which is to be the ventral surface of the embryo. 

 Whether this change of position is due to a shifting forward 

 of all the cells over the yolk, the axes of the ^gg remaining as 

 before, or whether it is only an apparent shifting due to an 

 alteration of the shape of the yolk, I am unable to say, but 

 incline towards the latter idea. It is also noticeable that the 

 segmentation hais now manifested itself at the surface, the sur- 

 face of the yolk being divided into areas corresponding to the 

 cells, a central mass of yolk, apparently destitute of protoplasm, 

 remaining unsegmented. In Jacra the superficial segmenta- 

 tion made itself manifest in the 32-celled stage; whether it 

 appears in the same stage in A. communis I am unable to 

 state, but it seems not improbable that it does. In A. aqnati- 

 cus, according to Van Beneden ('69), it appears in the i6-celled 

 stage. 



In the next stage 128 cells are formed, the D group having 

 now increased to eight (Fig. 33, D^-D^) and still remaining 

 distinguishable by their smaller size. The X group I was not 

 able to identify in this stage, at which, indeed, a very unfortu- 

 nate gap occurs in my observations. This is all the more 



