MITOSIS AND AMITOSIS. 399 



normal for a sufficient length of time under conditions of observa- 

 tion; furthermore, failure to observe amitosis in one hundred 

 such cells would still leave it possible that it might occur in a 

 second hundred. There is, however, a method by which the 

 occurrence or non-occurrence of amitosis can be determined with 

 great certainty and this is in the study of identical cell divisions 

 in hundreds and thousands of individuals. This is possible only 

 in the two maturation divisions which are so peculiar that they 

 can always be distinguished from each other and from all other 

 divisions, and in those early cleavages where the lineage of every 

 cell is known and its method of division can be observed in 

 thousands of different cases. If in every such dividing cell one 

 sees mitosis only, it can be concluded that this is the invariable 

 method of division in these cells. It is a significant fact, which 

 Child himself admits, that in no such case have the nucleus and 

 cell ever been seen to divide by amitosis, whereas in hundreds 

 and thousands of cases they have been seen to divide by mitosis. 



Nevertheless, in these very cells one frequently observes 

 lobulated, constricted or bipartite nuclei which might be regarded 

 as stages in amitosis were it not for the fact that the study of 

 the lineage of these cells shows that they invariably divide by 

 mitosis and that the peculiar shapes of the nuclei referred to 

 are due to modifications of normal mitosis. Such constricted or 

 bipartite nuclei occurring in cells the lineage of which was un- 

 known would naturally be mistaken for cases of amitosis and 

 it is very significant that the reports of amitosis in embryonic 

 cells have been invariably in cells of unknown lineage. 



This paper is based on a study of modified mitosis in the 

 maturation and cleavage of the eggs of the marine gasteropod, 

 Crepidula plana. A variety of nuclear forms are figured and 

 described which resemble more or less closely stages of amitosis 

 and yet it is perfectly certain that these nuclei divide only by 

 mitosis, and the manner of origin of these pseudo-amitotic forms 

 is plainly due to modifications of regular mitotic processes. 

 These modifications were produced by subjecting dividing cells 

 to abnormally high temperatures or to sea-water of abnormal 

 densities, but other similar modifications are produced by many 

 other abnormal conditions such as pressure, centrifugal force, 



