REPRODUCTION IN THE INFUSORIA. 191 



the relative positions they had at first assumed^ so that the 

 ventral margins of both which, when copulation commenced, 

 were exactly opposite one another, become quite everted, so 

 as to face in the same direction. But the pair of Oxy trichina 

 still remain united laterally for about one third of their 

 length, and that, moreover, in so complete a manner, that if 

 viewed in this region alone, no observer, ignorant of what 

 had previously taken place, would hesitate to acknowledge 

 that only a single organism lay before him. 



The length of time during which the sexual act continues, 

 may vary from twenty -four hours to five or six days, and 

 depends upon the degree of development at which the repro- 

 ductive organs have arrived, when the first symptoms of 

 union become manifest. Of this development, we shall now 

 proceed to trace the principal stages. 



Evolution of the Ovcwy and its contents. 



When first the ovary becomes observable in the body of a 

 young Infusorium^ it appears under the form of a colorless 

 transparent mass, with difficulty distinguished from the 

 parenchyma in which it lies as a somewhat paler and very 

 minute circular spot. By reason of its extreme softness it 

 cannot readily be examined out of the body, or submitted to 

 the influence of reagents. Dilute acetic acid shows its 

 apparently homogeneous tissue to be made up of numerous 

 pale granules, loosely united together by a colorless, gelati- 

 nous substance. The w^hole is invested by a delicate outer 

 membrane, and this, with the intermediate substance, soon 

 dissolves in the acid, whereupon the great molecules make 

 their escape. 



Not very dissimilar to this is the case of an adult animal- 

 cule, from which the fertilized eggs have been just extracted; 

 For a great change now takes place in the aspect of the 

 generative apparatus. In many Infusoria, in fact, it seems 

 to vanish altogether, soon however again to make its appear- 

 ance, though, at first, in a very rudimentary condition. Its 

 evolution now takes place with extreme rapidity, and is at 

 length consummated by the performance of another act of 

 reproduction. 



Such forms are far better suited than young Infusoria for 

 the study of the successive changes which the reproductive 

 apparatus undergoes, not only because in the former, these 

 parts are larger and more distinct, but they are also better able 

 to bear the action of reagents. Thus, a little acetic acid, brings 

 clearly into view the several structures above described as 



