192 PARASITISM 



the borders of the brood chamber, which is connected by broad spaces 

 with the peripheral rings of chromatin. From the walls of these rings 

 tubular ingrowths next develop and groAV down into the brood chamber 

 among the sporocysts (Fig. 81). When mature, and under proper 

 environmental conditions, not as yet recognized, these tubular 

 ingrowths are evaginated and the sporocysts ejected through them. 



(h) Sporulation in Coccidiidia. — In coccidiidia the processes of con- 

 jugation and sporulation are involved with complex sex differences, 

 pseudoconjugation, as observed in gregarines, being unknown. Here 

 a spermatozoid and an egg cell are formed and fusion is complete. 

 The fertilized cells, furthermore, have a somewhat different history 

 from those of the gregarines, where the zygote becomes at once the 

 sporoblast and secretes a single or double sporocyst. In the coccidian 

 forms, on the other hand, the fertilization nucleus of the zygote or 

 copula only rarely (Legerella, Mesnil) divides to form sporozoites 

 directly, but in the remaining genera the primary divisions give rise 

 to nuclei of two or more independent sporozoite-forming centres. 

 Thus, in Coccidium schnbergi the zygote nucleus divides twice, form- 

 ing four daughter nuclei, about which the protoplasm of the zygote 

 forms four sporoblasts. Each sporoblast secretes its own covering or 

 sporocyst, and each gives rise to two sporozoites (Fig. 74, p. 179). The 

 final mature germs are thus inclosed within two membranes, their own 

 sporocysts and the o5cyst which forms as a fertilization membrane, 

 Legerella alone being protected by the latter only. Classification of 

 the coccidiidia is frequently based upon the number of sporocysts 

 thus formed. Crijsfallospora crysiaUoides, like coccidium, has four 

 such sporocysts, but the great majority of the tetrasporocyst forms 

 belong to the latter genus. Others, notably cyclospora, diplospora, 

 and isospora, have only two sporocysts; still others, and perhaps the 

 most common forms, have more than four sporocysts, adelea, caryo- 

 tropha, and klossiella belonging to this category. 



In these forms, as in the gregarines, the number of sporozoites is 

 independent of the number of sporocysts; in barrousia, echinospora, 

 and diaspora (Fig. 20, p- 64) the sporocysts are monozoic; in adelea and 

 minchinia, dizoic; in benedenia, trizoic; in klossia, tetrazoic; and in 

 caryotropha, polyzoic. It is significant that in the malaria organisms 

 there are several centres of sporozoite formation, each of which, if 

 covered by a membrane, would be homologous with the polysporo- 

 cystid sporocyst. This may be merely a parallel development, or it 

 may have some phylogenetic significance, showing descent from coc- 

 cidium-like forms, with loss of the now useless protective sporoblast 

 membranes. 



(c) Sporulation in Myxosporidia. — Little more need be added to what 

 has already been given in connection with spore formation in this group 

 (see p. 143), for it is closely connected with the phenomena of fertiliza- 



