LIFE AND DEATH. 149 



so that the whole reproduction would depend on simple fission 

 alone during- the free state. 



The original mode of reproduction among the Monoplastides was 

 undoubtedly simple fission. This became connected with encyst- 

 ment, which originally took place without multiplication ; and only 

 when the divisions in the cyst became excessively numerous did 

 such minute plastids appear that a genuine process of development 

 had to be undergone in order to produce complete individuals. 

 Here we have the general conception of the germ as I defined it. 

 Its limitations are naturally not very sharply defined, for it is 

 impossible to draw an absolute distinction between simple growth 

 and true development accompanied by changes in form and 

 structure. For instance, Hackel's Protomyxa aurantiaca divides 

 within its cyst into numerous plastids, which might be spoken of 

 as germs. But the changes of form which they undergo before 

 they become young Protomyxae are very small, and for the most 

 part depend upon the expansion of the body, which existed in the 

 capsule as a contracted pear-shaped mass. It is therefore more 

 correct to speak only of the simple growth of the products of the 

 fission of the parent organism, and to look upon these products 

 as young Protomyxae rather than germs. On the other hand, the 

 young animals which creep out of the germs (the ' spores ') of 

 Gregarina gigantea, described by E. van Beneden, differ essentially 

 from the adult, and pass through a series of developmental stages 

 before they assume the characteristic form of a Gregarine. 



This is true development 1 . But such a method of germ-formation 

 and development are found most frequently, although not ex- 

 clusively, among the parasitic Monoplastides, and this fact alone 

 serves to indicate their secondary origin. It is a form of ontogenetic 

 development differing from that of the Polyplastides in that it does 

 not revert to a phyletically primitive condition of the species, but, 

 on the contrary, exhibits stages which first appear in the phyletic 



the number of divisions. This animal divides into at least a thousand daughter in- 

 dividuals. 



1 True development also takes place in the above-mentioned Ichthyoplithirius. 

 While in other Infusoria the products of fission exactly resemble the parent, in 

 IchthyopMhiriiis they have a different form ; the sucking mouth is wanting while 

 provisional clasping cilia are at first present. In this case therefore the word germ 

 may be rightly applied, and IchthyophtMrius affords an interesting example of the 

 phyletic origin of germs among the lower Flagellata and Gregarines. Cf. Fouquet, 

 'Arch. Zool. Experimentale,' Tom. V. p. 159. 1876. 



