THE SPOROZOA 219 



the actual fertilisation (Fig. 51, VIII ? ). At one spot the female 

 pronucleus approaches the surface of the macrogamete, and here a 

 " cone of reception " is formed as a little prominence of clear hyaline 

 protoplasm, from which a thin streak of similar substance extends 

 as far as the nucleus. A microgamete touches the cone and adheres 

 to it. The cone of reception is at once drawn in, and partly in this 

 way, partly by its own movements, the microgamete penetrates the 

 macrogamete, and its pronucleus reaches the female pronucleus. No 

 sooner is the entry of the microgamete completed than a clear 

 membrane, gradually increasing in distinctness, appears over the 

 whole surface of the zygote, excluding the less fortunate micro- 

 gametes, which die oft' and break up (Fig. 51, IX). The clear 

 membrane very soon becomes an exceedingly tough protective 

 envelope, the oocyst (ode), within which, after fusion of the two 

 pronuclei of the zygote, the sporogony runs its course. When the 

 oocyst is completely formed, the parasite is in a condition to abandon 

 the shelter of the host and to brave the outer world. The further 

 development can take place inside or outside the host, indifferently. 



It is apparent that the process of conjugation in the Coccidia bears 

 the greatest resemblance to the fertilisation by the sexual process in 

 animals and plants. Schaudinn has observed some curious points of 

 considerable interest in this process. Before the macrogamete has expelled 

 the fragments of its karyosome from the cytoplasm, it is not attractive to the 

 microgametes, but no sooner has it done so than they are drawn to it as 

 by a magnet. The attraction, which is evidently exerted by the substance 

 of the karyosome itself, acts very suddenly, and reaches from 48 p. to 

 130 p.. If exerted near to developing microgametes, it stimulates them 

 to great activity ; even still imperfect microgametes then develop 

 flagella, and in their struggles to free themselves from the micro- 

 gametocyte they carry away lumps of protoplasm with them. The 

 substance of the karyosome seems to be absorbed by the microgametes 

 that swarm to it, and the remarkable fact was observed that the attractive 

 power of the macrogamete was limited as regards the number of micro- 

 gametes drawn to it, the usual number being about twelve or fourteen. 

 When this number was made up, fresh microgametes approaching the 

 macrogamete were no longer attracted. After the substance of the 

 karyosome has dissolved up, a fresh attraction seems to be exerted by the 

 female pronucleus, which travels to the surface. 



The fusion of the two pronuclei in the zygote takes place in a very 

 remarkable manner. The female pronucleus passes back to the centre of 

 the zygote and becomes drawn out in the form of an elongated spindle, 

 on which the chromatin granules are arranged in parallel rows running 

 in a meridional direction (Fig. 51, IX). The male pronucleus, at first 

 huddled up at one pole of this nuclear spindle, also breaks up into granules 

 of chromatin, which mingle with those of the female pronucleus, and cause 

 the spindle to increase in size until it finally stretches through the entire 

 oocyst (Fig. 51, X). One pole of the spindle marks the point of entry of 



