474 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



the rounded cell. The nucleus at the same time becomes 

 invested in a proper capsule. After a period of repose, 

 the nucleus begins to break up into a number of 

 smaller pieces, the wall now breaks, and each nuclear 

 portion (spore) uniting itself with a certain quantity 

 of the surrounding protoplasm, separates from the rest 

 of the mass, escapes, and begins to grow up into the 

 form of the parent cell. 



Lastly, we sometimes observe that two individual 

 Protozoa of the same species become connected with 

 one another, the protoplasm of the two cells becomes 

 commingled, the whole quiescent, and invested in a 

 cyst. The enclosed contents break up into a number 

 of spores, which, on the eventual bursting of the cyst, 

 escape and begin to grow up into the form of their 

 parent. Here, then, not only is spore-formation pre- 

 ceded by encystation, but also by conjugation. At 

 the same time it is to be most carefully borne in mind 

 that the two individuals are, to all appearance, essen- 

 tially alike, and that there is no reason whatever for 

 regarding this conjugation as being a sexual act. 



The life-history of the Gregarina presents us with 

 a case of development by spore formation, which may 

 or may not be preceded by conjugation. A single 

 Gregarine, or two conjugated forms, become spherical, 

 and a firm structureless cyst is gradually developed 

 around the protoplasm ; the nucleus disappears, and 

 the whole of the contained mass breaks up into a 

 number of small separate bodies (spores) ; these are 

 often spindle-shaped, and from their occasional resem- 

 blance to the diatom Navicula, have obtained the dis- 

 tinctive name of psetidonavicellse. This appears 

 to be the most general mode of spore formation. The 

 spores become each invested in a distinct envelope, 

 within which the protoplasm is contained. The suc- 

 ceding stages of development have as yet been very 

 insufficiently studied ; in the large Gregarine of the 



