INTRODUCTION. 23 



before further fission is carried out, or not, does not 

 constitute an essential difference."* 



Calkins remarks that while the majority of the 

 Protozoa reproduce asexually, as described, reproduc- 

 tion in some is bound up with complete sexual differen- 

 tiation, and a series of forms may be selected which 

 indicate the probable development of the sexual from 

 the more primitive methods. In numerous cases the 

 sexual phenomena include many of the preliminary 

 maturation stages shown by the Metazoa, in the forma- 

 tion of polar bodies and reduction of the quantity of 

 chromatin, etc.f 



Blochmann asserted that " copulation," in which the 

 plasma-bodies of two animals become completely 

 fused together to form a new individual, as well as 

 "conjugation," in which the animals, after long-con- 

 tinued union, separate again from each other, and in 

 \vliich hitherto no demonstrable changes have been 

 observed, actually occur. 



In all cases of fission it is important to note the 

 part, in the process, which is taken by the nucleus. 

 Invariably the first signs of division are to be noted in 

 this organ ; the separation of the protoplasm, with its 

 contents, into two equal parts, follows, and one half of 

 the nucleus goes to complete the organisation of the new 

 individual. This is, briefly stated, the process observed 

 in the Amoebina. 



In the testaceous forms the process is essentially the 

 same. Here division takes place by the extrusion of 

 one half of the plasma-body through the mouth of the 

 shell. Some remarkable phenomena have been de- 

 scribed by Blochmann in the case of Euglypha alveolata. 

 The protoplasm emerged from the parent-shell, and 

 became covered with shell-lamellae, J forming a new 

 individual of the normal size, which received its moiety 

 of the divided nucleus. But, after this point had been 



* Art. "Protozoa" in 'Encycl. Brit.,' 9th ed. 

 t ' The Protozoa,' p. 55. 



J'Morphol. Jahrb.,' xiii (1887), p. 173. See Transl. in 'Ann. and Mag. 

 Nat Hist.' (6), vol. i (]888). 



