292 MEAD. [Vol. XIII. 



made which enable us to analyze the causes of these relations 

 into mechanical components. What is to be said at present 

 about these components must be deduced from a comparison of 

 normal cleavage in different forms; often where cleavage proc- 

 esses are similar, the mechanical conditions are differejit ; and, 

 where the conditions are similar, the processes are different. 



II. Cleavage Considered from the Point of View of 

 Developmental Mechanics. 



The doctrine of the mechanical causes of organic forms must, 

 when applied to annelid cleavage, account for (i) the relative 

 size of the cells, (2) the direction of the cleavage, and (3) the 

 time or rate of cleavage, for all three are potent factors in 

 determining the specific form of the trochophore. 



Relative size of the cells. — Aside from the very unequal 

 cleavage of the oocytes in the formation of the polar globules, 

 the first cleavage of the egg itself is sometimes equal, some- 

 times unequal. The explanation has been offered that the 

 inequality is due to the influence of yolk; that the karyo- 

 kinetic apparatus is not able to effect a cleavage through the 

 middle in cases where the yolk is abundant. A comparison of 

 Lepidonotus, which has little yolk and equal cleavage, with 

 Amphitrite and Clymenella, which have more yolk and unequal 

 cleavage, seems to sustain this explanation ; but a further com- 

 parison with other forms shows that the influence of yolk can- 

 not be considered the chief determining factor of the relative 

 size of cells. The ^gg of Chcetopteriis, for instance, is about the 

 size of that of Lepidonotus and has apparently the same amount 

 of yolk, yet it divides very unequally. The eggs of Crepidula 

 have a very lar^e amount of yolk and are much larger than 

 Ujiio, yet the first cleavage of the former is nearly equal, that 

 of the latter very unequal. In the later cleavage stages, cells 

 with a large proportion of yolk are as likely to divide equally 

 as unequally. Examples are numerous. In Amphitrite the 

 smaller of the first two blastomeres, though it arises by an 

 unequal division, divides equally, while each product of this 

 cleavage divides unequally. On the other hand, the largest 



