108 



THE SMALLEST LIVING THINGS 



fertilization membrane which becomes the outer capsule or sporo- 

 cyst of the fertilized cell (Fig. 63). The nucleus, formed by the 

 union of the male and female cells, or the amphinucleus of the 



Fig. 62— THE ASEXUAL 

 SPORULATION OF 

 EIMERIA STIEDAE 



An intestinal coccidian, parasitic 

 in the rabbit. Each coccidian 

 has divided into a number of 

 spores which appear somewhat 

 like a bunch of bananas. Each 

 spore is able to reproduce the 

 cycle 



From a photomicrograph by 

 the author 



Magnification, 700 



zygote, now divides twice, and cells form about the four nuclei 

 that result. Each of these four cells — called sporoblasts — now 

 secretes its own individual capsule, within which two germs, or 

 sporozoites, are formed by division. The sporocysts, with their 

 sporoblasts and eight sporozoites, are passed out with the feces 

 and sooner or later are eaten by other centipedes, and the history 

 is repeated. 



In such a life cycle, auto-infection, or spread of the infec- 

 tion in the same host, is brought about by asexual reproduction 

 of the parasite, while the spread of the infection among different 

 hosts is the result of sexual reproduction. There are thus two 

 cycles, one asexual, the other sexual, and these frequently occur in 

 different types of animal hosts. The asexual cycles of malaria 

 organisms, for example, occur in mammals and birds; the sexual 

 cycle in mosquitoes; the giant gregarine Porospora alternates 

 between lobsters and the mussel, Mytilus edulis; the coccidian 

 Aggregata alternates between crabs and cephalopod molluscs 

 (squid, cuttle-fish, etc.). 



Parasitism is thus an important branch of the science of 



