648 L. R. Cary, 



Part IV. 

 Oeiierîil Considerations. 



1. The nuclear division in D iplodis ens. 



The character of the nuclear division in this worm differs in 

 many respects from that which commonly occurs among the nietazoa, 

 and in just these points of difference it resembles the type of divi- 

 sion commonly found among- the protozoa. 



In the fact that the entire chromatin content of the nucleus is 

 gathered into a caryosome, the resting- nucleus of Diplodisctis re- 

 sembles that of many of the protozoa, especially the flagellata and 

 sporozoa. 



The persistent intranuclear position of the centrosomes, in the 

 segmentation divisions as well as in the maturation divisions, distin- 

 guishes the mitosis in this species from almost all instances among 

 metazoa. Among the protozoa, on the other hand, the mitosis in 

 KugJijpha alveolata, as described and figured by Schewiakoff (1888), 

 presents a series of nuclear phenomena which, except for the dis- 

 tribution of the chromatin in the resting nucleus and the formation 

 of the spireme, almost exactly duplicates that found in a segmen- 

 tation mitosis in Diplodisms. The persistence of the nuclear mem- 

 brane throughout the cell cycle, while a rather common occurence 

 among the protozoa, is unknown in the normal mitosis among metazoa, 

 and this fact makes a sharp distinction between the type found in 

 DiplocUscus and that found in other metazoa. 



A comparison of the figures given by many of the workers on 

 the development of the eggs from the parthenogenetic generations 

 of the Malacocotylea makes it seem probable that intranuclear mi- 

 tosis is of common occurence among the members of this family. 

 Most of the investigators above mentioned have made no accurate 

 study of the nuclear phenomena of division, but in many of their 

 figures chromosomes are shown within a sharply defined nuclear 

 membrane so constantly that it seems justifiable to conclude that 

 the mitosis in these species in similar to that occuring in Diplo- 

 cUscus. 



The evidence from figures of many celled embryos (germ balls) 

 is especially convincing, for in such cases the nuclei lie in a mass 

 of cytoplasm in which there are no cell walls so that there can be 



