72 THE PROTOZOA 



matin except by their respective relations to the life-cycle, at certain 

 periods of which, as will be seen, the nuclear apparatus is entirely 

 reconstituted, effete vegetative chromatin being eliminated from 

 the organism, either cast out or absorbed, and its place taken by 

 reserve generative chromatin. It is only necessary to remark that 

 some authorities speak of vegetative and generative chromatin as 

 if they were two distinct kinds of substance, whereas they are 

 probably to be considered rather as two phases or states of one and 

 the same chromatin. Vegetative chromatin is that which is in a 

 state of functional activity, and which thereby tends to become 

 exhausted and effete in its vital powers, exhibiting in consequence 

 the phenomena of " senility." Generative chromatin, on the con- 

 trary, by remaining inactive, conserves its ' youth ' unimpaired, 

 and constitutes a reserve from which the worn-out vegetative 

 chromatin can be replaced. Generative chromatin of one genera- 

 tion may become vegetative chromatin in the next. 



As regards their distribution in the cell-body, in some cases 

 vegetative and generative chromatin cannot be distinguished by 

 the observer as separate structural elements, but are mixed up 

 together in the same nucleus ; in other cases, however, they occupy 

 distinct situations in the body. Thus, in Sarcodina it is common 

 for the vegetative chromatin to be lodged in the principal nucleus 

 or nuclei, while the generative chromatin occurs in the form of 

 chromidia, as in Arcella (Fig. 32), or vice versa. In the Infusoria 

 there are two kinds of nuclei, which are shown by their behaviour 

 to consist, the one of vegetative, the other of generative chromatin. 

 Chromidia, when present in the cell, may also differ in kind, being 

 in some cases extrusions from the nucleus of purely vegetative 

 chromatin, in process of elimination, while in other cases, as 

 already mentioned, the chromidia, or a part of them, represent 

 the generative chromatin (see p. 150, infra). 



The nuclei of Protozoa exhibit great variety of structure and 

 form as compared with the relatively uniform structure of the 

 nuclei of Metazoa. As stated already, the constituent substances 

 or structural elements in any nucleus may be distinguished broadly 

 as chromatiriic and achromatinic : the former consisting of the 

 chromatin, the primary and essential element never absent in any 

 nucleus ; the latter comprising ^various accessory structures, an- 

 cillary to the chromatin, and not all of them invariably present 

 in any given nucleus. Amongst the principal achromatinic con- 

 stituents of nuclei in general must be mentioned the following : 

 (1) linin, occurring in the form of a framework, which s tarns feebly 

 or not at all by chroma tin-stains, and which presents the appear- 

 ance of a delicate reticulum or network, the optical expression of 

 an alveolar structure ; (2) a fluid enchylema or nuclear sap, filJing 



