THE PROTOZOA 137 



Life History. 



When it first reaches the sperm sac, Monocystis is a very 

 minute form, and it bores its way into a sperm mother cell becoming 

 an intracellular parasite. The sperm mother cell divides into a 

 number of daughter cells that become arranged to form a mulberry- 

 like mass, the sperm morula, around a central mass of non-nucleated 

 protoplasm, the cytophore. The young trophozoite lives in the 

 cytophore, which it gradually consumes. The cells of the morula, 

 after a certain number of divisions, give rise to numerous spermatozoa 

 which assume the typical form with a head and long thread-like 

 tail. By this time the parasite has consumed the central protoplasm, 

 and it now attacks the heads of the sperms, leaving the tails un- 

 touched. It is now fairly well grown, and, with all the tails of the 

 spermatozoa adhering to it, looks as if it were covered with a coating 

 of long cilia. For a while it lives in the spermatic fluid, thus 

 becoming an intercellular parasite. 



When full grown and mature two of these trophozoites come 

 together lengthwise and adhere. They shorten considerably and 

 secrete around themselves a very tough double-layered spherical 

 cyst, composed of a rigid outer coat or epicyst and a softer endocyst. 

 As these cells are destined to produce the gametes, they may now be 

 spoken of as the garnet ocytes. The nucleus in each passes through 

 a certain series of changes, resulting in the elimination of a quantity 

 of nuclear substance that is absorbed by the cytoplasm, and then 

 it undergoes repeated division, producing many daughter nuclei. 

 These take up a position around the periphery of the cell, and the 

 superficial cytoplasm breaks up into an equal number of small 

 masses, each enclosing a nucleus and attached to a central lump of 

 residual protoplasm. When they afterwards become free we -can 

 distinguish these minute nucleated bodies as the gametes, and they 

 soon enter a brief motile stage. The double layer of cuticle that 

 came between the two original gametocytes breaks down and the 

 gametes move about and come together in pairs, the two in each 

 couple most probably being derived from different parents. These 

 pairs and their nuclei fuse to form a single cell, the zygote or sporo- 

 blast. Each zygote secretes around itself a very tough resistant 

 cyst, the sporocyst, made of a substance allied to chitin and of a 

 characteristic elongated lemon shape. From its resemblance to 

 one of the unicellular plants, a diatom Navicella, in its tuin so named 

 from its boat shape, the sporocyst has long been known to naturalists 

 as the pseudonavicella. The nucleus of the sporoblast undergoes 

 three successive divisions, giving rise to eight daughter nuclei, which 

 take up a peripheral position near the middle of the length of the 



