The Protozoa 49 



the sporozoa form minute parasites of the red blood-corpuscles or 

 other cells of the vertebrates. The treponema is so small that it 

 can slowly find its way through the pores of a Berkefeld filter. 



On the other hand, the sarcoporidium is so large that one of its 

 cysts, composed of a single organism, can be seen with the naked 

 eye. Certain protozoa that play no part in morbid processes — 

 myxosporidia — and so do not come within the scope of this work, 

 may be several centimeters in diameter. 



Reproduction. — The reproduction of the protozoa takes place both 

 asexually and sexually. It may be that there are no strictly asexual 

 protozoa, nearly all forms having been shown upon intimate ac- 

 quaintance to be subject to occasional conjugation. Conjugation 

 may result in the loss of individual identity or the conjugated 

 individuals may again separate. 



Whether the reproduction takes place asexually without con- 

 jugation or sexually after conjugation, it always occurs by division, 

 which may be simple and binary or complex and multiple. 



Wherever a distinct nucleus can be found, the multiplication of the 

 protozoa is preceded by some kind of mitotic change. The more 

 complex the structure of the nucleus, the more complicated and 

 perfect the mitosis. 



The elongate protozoa divide lengthwise, which is sometimes 

 contrary to expectation, as in the cases of treponema and spirochaeta. 



The multitudinous sporozoites into which the zygotes of the 

 sporozoa divide are commonly the result of anterior division into 

 intermediate bodies known as oocysts, ookinetes, sporocysts, etc. 

 The nuclear substance is first divided so as to be uniformly dis- 

 tributed among these, then further divided so that some of it reaches 

 each sporozoite. 



In the process of sporulation the entire parent may be used up, 

 as in the coccidium and Plasmodium or the parent may continue to 

 live and later form additional sporozoites, as in sarcocystis. 



Encystment. — Nearly all of the protozoa are capable at times of 

 encysting themselves, i.e., surrounding themselves with dense 

 capsules by which life may be preserved for some time amid such 

 unfavorable surroundings as excessive cold, excessive dryness, and 

 absence of food. Sometimes the encysted stage is the spore stage 

 (coccidium), sometimes it is the adult stage (ameba).. Under these 

 circumstances we find an analogy with the sporulation of the 

 bacteria which is not for purposes of multiplication, but for self- 

 preservation. The encysted protozoa are less hardy, however, than 

 the bacterial and other plant spores, and succumb to comparatively 

 slight elevations of temperature. 



