The life history of Diplodiscus temporatus Stafford. 649 



no possibilit}' of confusing the outline of the nucleus with the boun- 

 daries of a cell. 



In the development of the fertilized egg, according to the obser- 

 vations of all modern investigators, the mitosis is similar to that in 

 other metazoa. In the later stages of the development of the sporo- 

 cysts of Diplodiscus the mitoses were of the same form as those 

 found throughout the later stages in the life history. In the earliest 

 cleavages of the fertilized egg it seems probable that the nuclear 

 membrane must be ruptured to allow the fusion of the male and 

 female elements. In view of this last mentioned fact, it would seem 

 probable that within the life cycle of a single worm there may be 

 both the intranuclear and extranuclear types of division : although 

 in any one species this has not been determined with certainty. 



In the fertilized eggs of the Heterocotylea, and in those of 

 Fasciola hepatica, at least, among the Malacocotjdea (Schübmann, 1905), 

 the centrosomes lie outside the nucleus. In most figures of the de- 

 velopment of the rediae and cercariae no mention is made of these 

 structures, but the apparently intranuclear type of the mitosis would 

 imply the intranuclear position of these bodies. 



The behavior of the chromosomes in the anaphase and in the 

 telophase of mitosis necessarily differs in Diplodiscus from what 

 usually takes place in metazoan nuclei. In this worm the chromo- 

 somes never form chromosomal vesicles, but fuse together to form 

 the dense caryosome, which, even with the best obtainable ditferen- 

 tiation, appears perfectly homogenons. from the late anaphase stage 

 to the beginning of its increase in size in the earlj^ prophase. In 

 this feature of the mitosis the resemblance to the mitosis in protozoa 

 is again very marked. 



2. The Life History of the Malacocotylea. 



In the light of the results recorded in this work the life history 

 of the Malacocotylea is shown to be one in which usually two 

 parthenogenetic generations are interpolated between any two gene- 

 rations coming from fertilized eggs. The old view that in the life 

 history of these forms there was a true alternation of generations 

 — Metagenesis — as well as that of Leuckaet, Balfour and Looss, 

 who saw in this process a metamorphosis extending over several genera- 

 tions, must be given up. Claus' interpretation of the life cycle as 

 heterogeny with paedogenesis seems to have the most facts in its 

 support. 



