298 MEAD. [Vol. XIII. 



porarily, but also those which suddenly stop dividing altogether. 

 For example, the cells of the primary prototroch in AvipJiitrite, 

 Clymenella and Nereis; the secondary prototroch cells in Amphi- 

 trite and Clymenella ; the " head-kidneys " of Nereis; the mucous 

 glands and paratroch cells in Amphitrite. These cells possess 

 no peculiarity of size, yolk segregation, nor position, which can 

 account for their sudden disinclination to divide. The neigh- 

 boring cells, which continue to divide, all share the same char- 

 acteristics, as far as size and yolk are concerned ; and, as for 

 position, in Chcetopterits the " prototrochal cells " do divide 

 again, — at least some of them. 



The conceptions of the mechanical interaction of cleavage 

 cells as units of mass are too crude to offer any explanation 

 of these complex phenomena. 



The cytogenetic origin of the prototroch and paratroch 

 forms an interesting study in cell differentiation, since these 

 organs are of great morphological significance and high physio- 

 logical specialization, and since their exact origin cell by cell 

 from the ovum has been accurately ascertained. Thus, for 

 example, if we apply a " theory of determinants " to these cases, 

 it would appear that the determinants of \h.Q pfimary prototivch 

 are all segregated in the four " trochoblasts " of the i6-cell 

 stage, for, though each of these four cells divides tzvice viore, 

 all the resulting cells are prototrocJial. 



The secondary trochoblasts {Aniphitj'ite and Clymenella), on 

 the contrary, contain other than prototrochal determinants. 

 Each divides into two cells, of which one contains only pu?-ely 

 prototrochal elements, while the other contains determinants of 

 other structures also, and its division results in three proto- 

 trochal cells and one cell which becomes part of the general 

 ectoderm. 



When we trace back the history of the determinants of the 

 primary and secondary trochoblasts respectively, we find that 

 they are already separated in the 8-cell stage. At this stage 

 the former are contained in the four upper, the latter in three 

 of the lower cells. When these determinants control certain 

 cells, even though the latter belong to different generations, 

 they all unite to form one organ, the prototroch. However, 



