382 HOLMES. [Vol. XVI. 



natural that the amount of food in the form of yolk should be 

 small, and the more nearly equal size of the blastomeres in the 

 early cleavage stages of pulmonates, in comparison with the 

 eggs of most marine forms, is probably due to the relatively 

 small amount of yolk in the ^gg. It is the rule among gastero- 

 pods that the greater the amount of yolk in the o^gg the smaller 

 are the ectomeres in relation to the entomeres. A comparison 

 of such yolk-laden eggs as those of Purpura and Nassa with the 

 eggs of Crepidula and Umbrella, or these again with the eggs 

 of Paludina or the pulmonates, will illustrate this principle very 

 forcibly. It has been pointed out by Kofoid, as a rule holding 

 for a great variety of forms, that " the greater the amount of 

 yolk, the greater seems to be the tendency of the cells of a 

 yolk-laden quartette to divide before those of the smaller quar- 

 tette." For instance, in the eggs of Limax, which have little 

 yolk, both macromeres and ectomeres at the eight-cell stage 

 divide almost simultaneously, and the &gg passes at once to 

 the sixteen-cell stage. In the eggs of Planorbis, Physa, and 

 Lymnaea, which contain somewhat more yolk, the macromeres 

 divide before the ectomeres, giving rise to a stage with twelve 

 cells. In the eggs of Umbrella and Urosalpinx, which contain 

 a still larger amount of yolk, even the third quartette is given 

 off before the first has divided. These facts can scarcely be 

 said to show, however, that the presence of yolk in cells actually 

 accelerates their division, as Kofoid seems to imply. It is a gen- 

 eral rule that the larger a cell is, the sooner it tends to divide. 

 There are numerous exceptions to this rule, some of which will 

 be pointed out later, but it expresses a more or less dominant 

 tendency in the cleavage of the egg. A large amount of yolk 

 in the ^gg, moreover, would determine the micromeres to be of 

 small size, and the small size of these cells would tend to delay 

 their cleavage. The yolk may, and probably does, delay the 

 cleavage of the cells containing it, but the small size of the 

 ectomeres in yolk-laden eggs delays their cleavage even more. 

 It is probably for this reason that we find a delayed cleavage of 

 the ectomeres in yolk-laden eggs. And this conclusion is in 

 harmony with the fact that the cleavage in yolk-laden eggs is 

 usually slow. 



