PEOTOZOA. 



87 



tions of sexual differentiation. They always propagate, therefore, 

 by modes wliicli are called asexual, among which, the chief part is 

 played by fission and generation. The nucleus appears to be of 

 great importance in all their modes of multiplication. 



Spores have been observed to be formed within the organism in 

 Rhizopoda. A larger or smaller part of the protoplasm of the body 

 is used in forming them ; when a larger portion is used this mode of 

 multiplication is allied to that mode which is so common among the 

 Protista, in which the whole body breaks up into a number of 

 spores, and so multiplies by division. In the Radiolaria the contents 

 of the central capsule are active in reproduction. The nuclei in 

 it become surrounded with protoplasm, and form flagellate swarm- 

 spores. 



The mode of reproduction is most exactly known in the Gre- 

 garinte. As a rule multiplication commences by the concrescence of 

 two individuals ; this generally occurs very early, so that the two 

 individuals, which form one body, the anterior end of one being 

 attached to the posterior end of the other (Fig. 29), 

 go on growing" for some time ; or conjugation 

 may only take place later, when the forms are 

 mature. After this comes a condition of rest, 

 accompanied by encystation, in which the two 

 individuals form a rounded body, with a partition 

 between them. Then the partition disappears, 

 and the substance of the body, and also the 

 nucleus, breaks up into an amorphous mass, from 

 which numerous vesicles gradually arise. From 

 these latter a number of germ corpuscles, called 

 " Pseudonavicellge,^^ on account of their shape, 

 are formed. These gradually fill the whole cyst, 

 and each gives rise to a single very small organism, 

 consisting of protoplasm solely, and this, being 

 without a nucleus, corresponds to a cytod. Each 

 of these structures moves about in an amoeboid 

 manner, and is gradually differentiated into a 

 young Gregarina, after which a nucleus is differentiated in its interior, 

 and it becomes limited externally by a cortical layer. 



Although conjugation has no exclusive signification in bringing 

 about these processes, as separate Gregarinas are also able to pass 

 through these reproductive processes in just the same way, yet it is 

 not the less important. It points, at least in the cases where it 

 exists, to the necessity of two individuals to bring about reproduc- 

 tion. It is, consequently, a pheenomenon preliminary to sexual 

 differentiation. 



Fig. 29. a h Two con- 



jugatedindividualsof 



Gregarina steniiridis. 



c Their nuclei. 



§ 70. 



Conjugation also plays a part in the reproductive proceedings of 

 Infusoria, for it is the first step in their multiplication. In this the 



