88 OUTLINES OF EVOLUTIONARY BIOLOGY 



which line the intestine of the host and upon which the parasites 

 feed (Fig. 39, A.). The latter increase in number very rapidly 

 by a kind of multiple fission, and successive generations of 

 parasites attack fresh epithelial cells of the host until the 

 epithelium is more or less completely destroyed. After many 

 generations have been produced asexually in this manner a 

 sexual process sets in. Megagainetes and microgametes are pro- 

 duced ; the former by growth of an ordinary individual into a 

 large spherical ovum, or egg-cell (Fig. 39, B., ? gam.), the latter 

 by growth and division of an ordinary individual into a number 



<?.r 

 nu. 



A."* D 



FlG. 39. Coccidium schubergi, highly magnified. (After Scliaudinn.) 



A. A full grown Coccidium lying within an epithelial cell of the host. 



B. Male and female gametes about to conjugate. 



coc., the Coccidium ; c.r., the cone of reception put out by the female gamete ; TIM., nucleus 

 of female gamete ; nu.coc., nucleus of adult Coccidium ; nu.ep., nucleus of epithelial 

 cell of host ; g gam., male gamete or spermatozoon ; J gam., female gamete or ovum. 



of very much smaller spermatozoa or sperm cells (Fig. 39, B., 

 J gam.). 



The ovum is densely filled with food granules and has no 

 power of locomotion. The spermatozoon closely resembles a 

 flagellate monad, being provided with a pair of flagella by means 

 of which it swims actively about ; the body, however, is long and 

 slender and consists almost entirely of nuclear (chromatin) material. 

 It seeks out the ovum, which exercises a peculiar attraction upon 

 it, and the two conjugate, the spermatozoon boring its way into 

 the ovum and their nuclei fusing to form the zygote nucleus. 

 The only trace of activity, apart from nuclear phenomena, which 

 the ovum exhibits is the protrusion of a small "cone of recep- 

 tion " (Fig. 39, B. c.?-.) from the surface of the cell towards the 

 approaching spermatozoon, which seems to indicate that the 



