244 °- C - GLASER. 



shows at least that these must be rare. The same thing may be 

 said of embryonic tissues in general, as Child has emphasized. 

 Who has not been struck by the comparative scarcity of mitosis 

 in tissues which are known to grow with great speed ? 



As implied in the introduction to this paper, whether amitosis 

 plays a part in normal cell-differentiation, and whether direct 

 divisions may intervene between indirect ones, without inhibiting 

 further differentiation are really two distinct questions. In prac- 

 tice however it is impossible to keep them separate, for if ami- 

 tosis does play a role, it does this in a normal tissue, and it is 

 characteristic of normal tissues that their component cells at some 

 time exhibit mitosis. The results both of Meves and of Wheeler 

 offer cases in point. The same is true of Child's work. In 

 Moniesia also cells which are part of an apparently normal cycle 

 divide at one time amitotically (oogonial and spermatogonial 

 divisions) and later mitotically (maturation divisions). Similarly 

 after fertilization, the first cleavage of the egg is accompanied by 

 a typical mitosis, whereas the later cleavages may be amitotic. 

 Since the cells of the cleavage period are the ones from which 

 the definitive structures of the adult come, it follows that amitosis 

 plays a part in normal cell differentiation. 



Neither Mo?iiezia, Child's form, nor Fasciolaria are ideal ani- 

 mals to work upon, for aside from the mere matters of technique 

 which in one of them offer considerable difficulty, both of these 

 forms exhibit in the tissues studied (entoderm ; ovary ; testis) 

 degenerating cells, and a certain number of mitotic divisions along 

 with the amitotic ones. The possibility therefore exists that the 

 indirect divisions are the really important ones, whereas the ami- 

 toses are physiological, and of no consequence in a genetic sense. 

 According to Wheeler Blatta must be ideal for " all of the future 

 divisions of the blastoderm and those subsequently undergone by 

 the serosa are akinetic." Apparently here there is no chance 

 of a mistake. In the absence of other forms equally well adapted 

 for our purposes, there is only one thing to do — to measure as 

 accurately as possible the frequency of the direct and indirect 

 divisions in a tissue, and then on the basis of these measurements 

 to see if the facts of growth that need explanation can be explained 

 when one or the other of the two forms of division is ruled out. 



