196 PROCEEDINGS OF THE AMERICAN ACADEMY. 



changes induced in the position of the quartets by the accumu- 

 lation of yolk in the basal quartet, the cells in ISTeritina retain 

 the same nomenclature that is given to the homologous cells of a 

 form such as Limax, presenting the typical undisturbed super- 

 position of quartets. 



In all the literature examined there occurs but a single case 

 where the position of an indicated spindle, contradicts the princi- 

 ple of alternation of spirals. This is found in Lang ('84, Taf. 34, 

 Fig. 20) in the division of the cells aey-de^ forming ae x -de x and 

 ae 6 - de 6 . Little can be said with regard to this except that we 

 do not know the full nuclear history of this cleavage, and that it 

 occurs at the close of the spiral period, immediately over the 

 group of " Scheitelzellen " that has just sunk below the level of 

 the remaining ectoderm. This may have produced secondary 

 modifications in the direction of the spiral. 



It remains for me to speak of the influence of yolk upon cleav- 

 age. Reference to Plate I. Fig. 2, shows that in Limax the cell b 53 

 belonging to the quartet of this generation, which is the last to 

 divide, is a member of the smallest quartet of the fifth generation. 

 It will further be seen that the order of the division of the quar- 

 tets, viz. first, second, fourth, third, is also the order of size from 

 largest to smallest, the quartet of largest cells, i. e. those presum- 

 ably with the greatest amount of yolk being the first to divide 

 in this generation, as it was also in the preceding generation. 

 This phenomenon is by no means confined to Limax. It is found 

 in a great variety of forms, and the greater the amount of the yolk 

 the greater seems to be the tendency of the cells of the yolk-laden 

 quartet to divide before those of the smaller quartets. This can 

 be illustrated by a comparison of Figure 1 (Plate I.), which shows 

 the egg of Limax approaching the sixteen-cell stage, and Figure 7 

 (Plate II.), which shows ISTeritina approaching the same stage. In 

 Limax there is little yolk, and the cleavage of the two quartets 

 is almost coincident. In Neritina, however, where the yolk is 

 more abundant, the lower quartet has divided, and its nuclei have 

 assumed a " resting condition " before the cleavage of the upper 

 quartet has fairly begun. Thus we have a well marked twelve- 

 cell stage. The same tendency of the yolk-laden cells to cleave 

 before the others is seen in the later stages of ISTeritina, and in- 

 deed in many forms of Molluscan cleavage. It is also found in 

 Nereis and Discoccelis. It is not, however, confined to the spiral 



