No. 2.] EARLY DEVELOPMENT OF PLANORBIS. 383 



As, perhaps, in all other mollusks, except the cephalopods, 

 and in the annelid worms, all of the ectoderm is contained in 

 the first three quartettes of micromeres. The three macro- 

 meres, A, B, and C, are entirely entodermic, while the poste- 

 rior one, D, contains both entoderm and mesoderm. The cases 

 in which more than three quartettes of ectomeres are said to be 

 formed I think we must regard, with Conklin, as open to seri- 

 ous question. In the case of Fulgur, in which a large number 

 of quartettes of ectomeres was said to be formed, Conklin has 

 shown that there is, in reality, only the usual number, three. 

 In Nassa, Bobretzky held that the macromeres budded off a 

 large number of ectomeres ; but Conklin finds that in the closely 

 related genus, Ilyanassa, the usual number of quartettes is pro- 

 duced. Salensky has asserted that more than three quartettes 

 of ectomeres are formed in Vermetus, and Erlanger has made 

 the same statement regarding Bythinia. But in Serpulorbis 

 squamata, from the coast of California, — a form very closely 

 allied to Vermetus, in which genus it was, in fact, originally 

 placed, — I have found that the whole ectoderm arises from 

 but three quartettes. 



It is not so much the fact that the cases reported of the 

 formation of more than three quartettes of ectomeres are 

 exceptions to a general rule that makes them so improbable — 

 it is the very definite and similar fate of these quartettes in all 

 the forms in which their history has been traced. Speaking of 

 this rule that the ectoderm in annelids and mollusks arises from 

 three quartettes. Dr. Conklin says, "The cause of this remark- 

 able phenomenon is to be found in the fact, I believe, that each 

 of these quartettes of ectomeres is the protoblast of definite 

 regions of the embryo." Each quartette forms essentially the 

 same parts of the embryo in every mollusk that has been studied 

 in this regard. An additional quartette would necessitate a con- 

 siderable modification of the fates of the preceding quartettes. 

 Were the fates of the different cell generations to a large extent 

 indeterminate, a variation in their number would not seem so 

 great an improbability. Since, however, each quartette has essen- 

 tially the same destiny, not only in widely separated groups of 

 the Mollusca but also in annelids, we have very strong reasons. 



