No. 3.] PODARKE OBSCURA VERRILL. 445 



one of which the egg chooses before it develops much 

 farther. We might assume that in the later cleavages a differ- 

 entiation suddenly appears, e.g., at the 64-cell stage, so that 

 from that time on bilateral divisions occur. I know at present 

 of no way by which we could prove or disprove this supposition. 

 From a careful study of the Q^%, however, I do not believe that 

 such is the case. At the 64-cell stage and later, marked dif- 

 ferentiations are evident ; but it is to my mind much more 

 reasonable to suppose that a differentiation has been present 

 from the beginning than that it suddenly appears at this stage. 

 I do not mean by this that the material is completely marked 

 off in the unsegmented ovum, but that in Podarke, as in Amphi- 

 trite, a gradual differentiation appears as development pro- 

 gresses, and that the process is essentially the same in the 

 two cases. 



The important differences to be explained between the cleav- 

 age of Podarke and forms with equal cleavage are these : 

 I. In Amphitrite, Nereis, Arenicola, etc., D is larger than 

 either A, B, or C ; in Podarke all four cells are equal, 2. In 

 Amphitrite, etc., the first generation of ectomeres is smaller 

 than the cells below them ; in Podarke they are all alike. 

 3. In Amphitrite, etc., 2d is much larger than either of the 

 other three members of the same quartette ; in Podarke they 

 are all of the same size. 4. In Amphitrite, etc., 4d is much 

 larger than 4c, 4b, or 4a ; in Podarke again they are of the 

 same size. 



I have given above my reasons for assuming that lack of 

 differentiation is not responsible for these size differences, and 

 in two previous articles (Nos. 29, a and 29, b) I have attempted 

 to explain them by applying the principle of "precocious 

 segregation " proposed by Lillie and modified by Conklin. In 

 other words, I believe that the equal cleavage of Podarke is 

 due to the fact that in the cell D there is accumulated a 

 smaller amount of material destined to produce certain 

 definite parts of the body than in those forms with the 

 unequal type. 



From the cell D first arises 2d, which in Amphitrite gives 

 rise to the entire ectoderm of the body behind the first somite, 



