No. 2.] DEVELOPMENT OF MARINE ANNELIDS. 297 



When the cleavage is once started, it is evident that the 

 form of the embryo is conditioned by the relative rapidity of cell 

 divisions in certain regions, as well as by the relative size of 

 the cells and the direction of cleavage. 



A close comparison of the rate of cleavage of corresponding 

 cells in the eggs of various annelids, molluscs, etc., and of the 

 rate in different cells in adjacent areas of the same egg, clearly 

 demonstrates that neither the effect of gravity, the size of the 

 cells, their position, nor all these together are sufficient to account 

 for the rate of cleavage. As to the effect of gravity, it is only 

 necessary to repeat that many eggs develop normally in any 

 position. Let us test the other factors in a number of instances. 

 In Nereis the smaller of the two unequal blastomeres regu- 

 larly divides first ; in ChcBtoptenis, on the other hand, though 

 unequal, both blastomeres divide at exactly the same time. I 

 have examined this Qgg with particular care by means of sec- 

 tions, and find that at successive stages of karyokinesis even 

 the daughter centrosomes at the ends of the spindles are just 

 as far apart in one cell as in the other. In Amphitrite, also, 

 the two cells divide at about the same time, — sometimes, 

 however, the larger one is slightly in advance. 



In many forms the division of the first eight cells takes 

 place synchronously, whether the cells are of nearly the same 

 size and contain little yolk, as in Lepidonotus, Eiipomatiis, 

 etc., or whether, as in many other forms, of which Scolecolepis 

 is perhaps the best example, the four upper cells are compara- 

 tively small and free from yolk, while the four lower cells 

 are large and full of yolk. 



The mesoderm cell, in several forms, manifests great karyo- 

 kinetic activity, while the entoderm cells, which correspond 

 exactly in origin, but lie in the other quadrants, cease dividing 

 temporarily, — yet in size and amount of yolk the mesoderm cell 

 often very closely resembles those of the entoderm. The cells 

 of the upper hemisphere in Unio are tardy in division, but are 

 of moderate size and, to all appearances, similar to the corre- 

 sponding cells of Amphitrite, Nereis, etc. 



The consideration of the rate of cleavage must include not 

 only cells which are precocious or tardy, or cease to divide tern- 



