78 PRACTICAL PARASITOLOGY 



formed and these contain sporozoites within a somewhat resistant 

 envelope. 



The finer structural details of the plasm and nucleus are best 

 observed in cover-glass preparations, which should be fixed in 

 alcoholic solution of mercuric chloride and stained with hsematoxylin. 

 The manner in which the bladder-shaped nucleus becomes dissolved 

 after encystment is seen in quite young cysts. The greater part of 

 the nucleus, with the exception of the caryosome, perishes. It 

 becomes broken up and absorbed by the plasma, a small portion of the 

 vegetative nucleus being used to form a sex-nucleus, which divides 

 repeatedly by mitosis. The more minute details of these nuclear 

 changes are, however, only to be seen in sections. The pseudo- 

 navicellse, moreover, do not stain well in cover-glass specimens, even 

 though the utmost care be taken to spread the cyst contents in the 

 thinnest possible smear upon the glass. 



Sections are cut from the entire seminal vesicle, which should be 

 fixed whole in alcoholic solution of mercuric chloride. They are best 

 stained with hsematoxylin and should be counter-stained with eosin ; 

 or iron-hsematoxylin may be used. In sections prepared in this 

 manner, the further stages of the reproductive process may be 

 observed. In both Gregarines numerous little nuclei, which are the 

 ultimate products of repeated division, wander to the periphery of the 

 parent, where they each surround themselves with a layer of denser 

 plasma. At the same time, the principal part of the plasmic body of 

 both parents becomes markedly alveolar and undergoes obvious 

 degeneration. In a later stage, the little daughter-cells become com- 

 pletely separated from the central plasmic body. These are the 

 gametes, the slightly dissimilar offspring of the two parent Gregarines 

 which originally combined to form a common cyst. The gametes of 

 the one parent are a little larger than those of the other, and they 

 contain a somewhat larger nucleus, which is, however, not quite so 

 rich in chromatic contents as that of the smaller gametes. These 

 gametes then fuse in pairs ; and as the process commences with the 

 fusion of the plasmic substance, the nuclei remaining separate for a 

 time it is easy to see, by the difference in size of the two nuclei, that 

 copulation always takes place between two dissimilar gametes, the 

 offspring in the first instance of different parents (fig. 22). 



In the Gregarines of the earthworm, the difference between the 

 two gamete forms is so slight that it requires the most careful observa- 

 tion to distinguish them. Thus in one species, the smaller gametes 

 measure 3%5 JJL in width and 4 //, in length, their nuclei having a 

 diameter of 1*5 y^, while the measurements of the larger gametes are 

 4*5 //,, 5./U-, and 2 /u, respectively. In certain polycystic varieties the 

 sex dimorphism is much more pronounced. 



