88 



COMPARATIVE ANATOMY." 



nucleus is of considerable importance; it (Fig. 30, n) is a firm 

 structure, sometimes provided with a spiral envelope, very various 



in form. It lies in the cortical 

 substance of the body, or is 

 surrounded by a continuation 

 of this substance, if it is more 

 deeply sunk in the interior. It 

 is sometimes oval or round, or 

 it is flattened and curved (Vor- 

 ticellinge), or it is even greatly 

 elongated and regularly con- 

 stricted (Spirostomum). The 

 importance of the nucleolus, 

 which differs from the nucleus 

 in nothing but its smaller size, 

 is more obscure. The act of 

 reproduction commences as a 

 rule with the complete or partial 

 fusion of two individuals, which 

 may be of the same or of dif- 

 ferent sizes ; this fact led to 

 the mistaking of conjugation 

 for stages of fission or gemma- 

 tion. This concrescence gives 

 the stimulus to changes in the 

 appropriate parts. The nucleus 

 becomes divided into a certain 

 number of parts, around which the protoplasm is disposed. In 

 this way a number of spores are formed, each of which becomes a 

 new individual while within the mother-cell; and then gets an 

 investment of cilia, and escapes to the exterior. 



It is still a question as to the share which the nucleolus takes in 

 this process ; and the statement that in one group of the Ciliata it 

 has the function of a sperm-forming organ, while the nucleus has 

 the function of an ovary, requires to be confirmed. In any case this 

 differentiation of a male organ is not a common phenomenon, but is 

 limited to a very narrow circle. The nucleus, therefore, and the 

 nucleus alone, is certainly known to take an active share in repro- 

 duction, and this share is of just the same character as that which it 

 was seen above to have in spore formation, and as it has in gemma- 

 tion, in which the nucleus of the bud has often been observed to 

 arise from the previous gemmation of the nucleus of the mother-cell 

 (Podophrya). Finally, multiplication by fission is very common, 

 although conjugation was often confounded with this process at one 

 time. 



Fig. 30. — An Acineta with part of its 

 stalk, p Psenclopodia-like, but stiff, ten- 

 tacles, v Vacuoles, n Nucleus, e Aciliated 

 young form lying in the so-called broad 

 cavity. 



