PROTOZOA. 821 



pulverisation, i.e. is disseminated in a minutely divided state through the proto- 

 plasm. See on the forms of nuclei R. Hertwig, M. J. ii. 1876, and on the same 

 with their modes of fission Gruber, Z. W. Z. xxxviii. 1883, and pp. 834-5 infra, on 

 the Infusorian nuclei. 



Reproduction takes place by fission, gemmation and spore-formation. The 

 distinction between the two modes first named depends solely on the relative size 

 of the parts : if they are equal or sub-equal then the process is termed fission ; if 

 markedly unequal, it is gemmation. Where axes are distinguishable in the organism, 

 fission is termed transverse, longitudinal or oblique with reference to the longest 

 axis. It is usually binary, i. e. the organism divides into two ; it is very rarely 

 multiple, i. e. the organism is resolved simultaneously into several parts ; but it is 

 frequently repeated without a pause of any length, leading to a more or less rapid 

 diminution of size. Gemmation may be external, i. e. the bud projects freely, or 

 internal, i. e. the bud is developed within a closed or nearly closed depression (some 

 Aanetaria). One bud may be formed at a time or several. The term spore is 

 applied to two different classes of structures : to bodies which are produced by the 

 condensation of the protoplasm accompanied or not by fission, and which pass 

 through a resting phase : or to bodies produced by the progressive or simultaneou s 

 total or partial resolution of the protoplasm into ultimately small portions, which are 

 not set free until the process is complete, which do not resemble the parent 

 organism when they are set free, and which as a rule pass through a quiescent 

 period. The spores, however produced, may be naked, or protected by a special 

 spore-membrane and may then be distinguished as chlamydospores. They may 

 when they become motile be amoeboid or flagellate, and to these two states 

 respectively the terms amoebula, or zoospore s. flagellula may be applied. In fission 

 the nucleus divides before or after the commencement of division in the body of the 

 cell ; the same is true of gemmation. Indeed it has been shown experimentally in 

 Infusoria by means of artificial section, that the presence of a part of the nucleus is 

 indispensable to the progress of normal life. Non-nucleated fragments may increase 

 in size and heal a wound, and if an organ such as the peristome in an Infusorian 

 be in process of development at the time of section, it undergoes complete evolu- 

 tion ; but such fragments cannot start the formation of an organ themselves ; their 

 life is limited, nor are they capable of reproduction. See on this subject Gruber, 

 A. N. H. (5), xvii. 1886, p. 473; Nussbaum, A. M. A. xxvi. 1886, p. 509. As to 

 spore-formation or sporulation, in some instances, e. g. Radiolaria, Foraminifera, it 

 has been clearly proved that a multiplication of the nucleus precedes the resolution 

 of the protoplasm. 



A spontaneous breaking up of the body which is independent of the nucleus 

 occurs in some Infusoria. It is of normal occurrence in the multinucleate Opalina^ 

 where it 'leads to the formation of new individuals. This, however, is probably not 

 invariably the case ; and the process is one of destruction. See Gruber, op. cit. 

 supra, p. 481 ; Saville Kent, Manual of the Infusoria, p. 84; Parker, on Amphi- 

 leptus, A. N. H. (5), xiii. 1884, p. 416. 



Inasmuch as a Protozoon multiplies itself solely by some mode of cell-division, 

 and the process continues under normal circumstances in a recurrent manner, 

 it follows that there is no loss of substance at any time similar to the physical de- 

 struction or death which inevitably overtakes multicellular animals, i. e. all parts 

 save the generative products, by which the race is perpetuated. For the discussions 



