84 



Messrs. C Dobell and A. P. Jameson. 



sporoblasts. The uninucleate sporoblasts are then converted into spores, 

 within each of which three uninucleate sporozoites are finally formed by 

 further divisions of the nucleus and differentiation of the cytoplasm. In the 

 asexual cycle, the sporozoites, after escaping from the spores, grow into large 

 schizonts. By repeated divisions of the nucleus, each schizont becomes 

 multinucleate, and finally breaks up into a very large number of uninucleate 

 merozoites. When the latter leave the crab and enter the body of the cuttle- 

 fish, they grow into the male and female individuals of the sexual cycle — thus 

 completing the life-history.* 



We may begin the history of the chromosomes with a description of these 

 bodies in the male parasite. During the period of growth, the nucleus passes 

 through a series of very complex stages which need not be described here. 

 At the end of this period the first nuclear division takes place. This division 

 begins as an ordinary mitosis, but ends as a multiple mitosis of a peculiar 

 type. In the prophases, six filamentar chromosomes are formed from the 

 spireme. At diakinesis they can be clearly seen and counted. They consist 

 (fig. 1, A) of one very long chromosome (a) and one very short chromosome 

 (/), the remaining four (b-e) forming a regular series of intermediate sizes. 

 During the period of the first division, the karyosome disintegrates and 

 disappears. It plays no part in the formation of the chromosomes. 



As the chromosomes pass on to the equatorial plate of the first spindle, 

 they shorten and thicken until they become almost spherical. They 

 preserve, however, at this arid all subsequent stages, their characteristic size- 

 relations to one another (fig. 1, B, a-f). At the metaphase they divide by 

 constriction, a daughter-group of six differentiated chromosomes passing to 

 each pole of the spindle (fig. 1, C). In the late anaphases the chromo- 

 somes again become filamentar. The asters at the poles of the spindle 

 divide many times in succession, and each time all the chromosomes split 

 longitudinally. A complicated polyaster figure is thus formed, from which 

 the chromosomes finally emerge in groups of six (fig. 1, D, a-f). Each group 

 enters into the formation of a resting nucleus at the periphery of the organism. 

 After this first multiple division, the nuclei divide many times in succession 

 by ordinary bipolar mitosis — the typical set of six chromosomes being 

 recognisable at each division. The smallest nuclei finally formed enter into 

 the microgametes. No halving of the chromosomes takes place, therefore, in 

 the formation of these. Each microgamete nucleus receives a typical set of 

 six chromosomes, like those formed for the first division. 



The macrogamete is formed, in the typical coccidian manner, by each 

 female individual being transformed into a single gamete. The nucleus of 

 * See Siedlecki (1898), Leger and Duboscq (1908), Dobell (1914). 



