86 



Messrs. C. Dobell and A. P. Jameson. 



the female passes through a complex series of stages similar to those 

 occurring in the male ; and these, likewise, culminate in the formation of 

 chromosomes, though no nuclear division takes place before fertilization. 

 Here, again, the chromosomes can be clearly counted, and consist of six 

 filaments differing in size from one another (fig. 1, E, a-f) exactly as in the 

 male. The karyosome breaks up and disintegrates ; and, as in the male, 

 plays no part in forming the chromosomes. 



Before fertilization the chromosomes break up within the nucleus of the 

 macrogamete. The microgamete then enters, and a series of very complex 

 nuclear changes ensues. These include the formation of a so-called 

 " fertilization spindle " characteristic of the Coccidia. At the end of these 

 changes the zygote nucleus divides — the division beginning like an ordinary 

 mitosis and then becoming multiple, as in the first division in the male. 

 The behaviour of the chromosomes during this division is very remarkable. 

 From the spireme 12 chromosomes are formed, which can be clearly counted 

 at diakinesis (fig. 1, F). They consist of two homologous sets of six chromo- 

 somes each — each set (a-f, a'-f) consisting of six filaments differentiated as 

 regards length from one another. The two sets clearly represent the two 

 groups of chromosomes which the zygote nucleus has received from the male 

 and female pronuclei. 



When the achromatic spindle has been formed for the first division of the 

 zygote nucleus, the 12 chromosomes pass on to its equator. As they do so 

 they shorten and thicken until they become almost globular. At the same 

 time they associate in homologous pairs — a with a',f with /', etc. — so that 

 six bivalent or double chromosomes finally come to lie on the equatorial 

 plate (fig. 1, G-). At the metaphase, disjunction of the temporarily united 

 chromosomes takes place, without any splitting of the individual chromo- 

 somes ; so that a group of six differentiated chromosomes passes towards 

 each pole of the spindle (fig. 1, H). The rest of this division resembles the 

 first division in the male. The asters divide, and the chromosomes become 

 filamentar and split lengthwise. From the polyaster figure so formed the 

 chromosomes emerge finally in sets of six (lig. 1, K) and enter into the 

 formation of as many resting nuclei as there are sets of chromosomes. 



The subsequent nuclear divisions of the zygote all take place by ordinary 

 bipolar mitosis. At each division a typical set of six chromosomes (fig. 1, L) 

 is present — cue such set entering, therefore, into the nucleus of each 

 sporoblast. A similar set of six chromosomes can be counted at each of the 

 two mitotic divisions which take place within the spore to form the 

 sporozoite nuclei (fig. 1, M). 



During the development of the sporozoite into a schizont, the nucleus 



