22 University of California PuMications in Zoology [Vol. 18 



themselves into only three types. The first, that of the "single indi- 

 vidual" cysts (pi. 1, fig. 12), the second, "binary" cysts (pi. 1, figs. 

 13, 16), and the last the "multinucleate" cysts (pi. 1, fig. 14). The 

 binary cyst is in all probability the same as the so-called ' ' copulation ' ' 

 cyst ; in the latter case complete separation of the cytoplasmic bodies 

 has occurred, while in the former the cleavage of the parent body is 

 still in process. 



In the results of my study of the "multinucleate" cysts there are 

 no grounds for attributing maturation phenomena to the parasite 

 after encystment. No evidence was found which could be interpreted 

 as progressive fusion of two individuals in a cyst ; and when nuclei 

 appeared larger, it was because they had another division to undergo ; 

 furthermore, no evidence for a reduction of the four ancestral chromo- 

 somes to two chromosomes was ever found ; the chromatin content 

 appeared equal for all the sixteen nuclei. 



The sixteen nuclei came, therefore, as a result of multiple fission, 

 three progressive divisions having taken place. 



Conclusions 



1. Mitosis in Giardia microti presents phases characteristic of 

 mitosis in Metazoa, viz., prophase, metaphase, anaphase and telophase. 

 The normal number of chromosomes is four. 



2. The nuclear membrane of each nucleus persists during the 

 entire process of mitosis. 



3. The prophase presents many peculiar and complex nuclear 

 changes : 



(a) The karyosome elongates to form a single spireme band sup- 

 ported by a linin-like substance in the main axis of the nucleus. 



(h) The single spireme band in each nucleus splits longitudinally 

 from the centrosome distally to form two spireme strands. 



(c) Each spireme strand segments to form four chromosomes. 

 The segmentation proceeds from the region of the centrosome poste- 

 riorly, due to the probable influence of the centrosome. 



(d) Through all these stages the chromatin in its distribution on 

 the spireme exhibits a polarity related to that of the body of the flagel- 

 late ; the long axis of the chromosomes also shows this polarity. 



(e) The eight chromosomes formed appear at first in two groups 

 of four chromosomes each, one group above the equator of the nucleus 

 and the other group below, suggesting biparental origin. 



