OF THE PHTTOZOA. 123 



naturalists — for instance, by Weisse in Cldorogonmm, and by Perty in each 

 family of Phytozoa. The production of such bodies is frequently treated of 

 as development by germs, and, no doubt, is the same phenomenon Ehi^enberg 

 represents as \dviparous reproduction. 



Microgonidia are not so commonly developed as macrogonidia ; and indeed 

 their formation would seem determined, at times at least, by external cir- 

 cumstances affecting their functions and vital activity unfavom-ably. Thus, 

 Cohn (Entiv. p. 168) narrates the circumstance of the peculiar and pretty 

 general development of microgonidia, in Clilcnmjdococcus, after a thunder- 

 storm. 



Peocess of Encystln'g : Coxdition of rest. — The perpetuation of Phytozoa 

 is provided for, as before intimated, by another process, which both secures 

 to the cells imdergoing it a power of successfully resisting influences that 

 to unprotected gonidia are destructive, and is connected with an ulterior act 

 of development. This faculty of self-protection is called " the encysting pro- 

 cess," since by it the cell encloses itself within an additional fii-m timic, which 

 suiTounds it like a case or cyst, and transforms it into a 'still' or 'whiter ' spore. 



The process takes place in all the Phytozoa after the same fashion : the 

 protoplasmic covering of the gonidium or cell secretes around it a dense, firm 

 envelope, which in general becomes raised from it all round, so as to leave a 

 clear intervening space. On the assumption of this extra coveiing, cells 

 previously motile and active enter on the ' still ' condition and lose their 

 ciha ; at the same time, the character of the contents is altered, and a red 

 colour frequently acquii^ed. The transformation in their physical structm^e 

 is accompanied by a physiological change ; for in place of seeking the light, 

 exhahng oxygen, and carrying on all the vital processes with a corresponding 

 activity, they sink to the bottom and conceal themselves from the light. It 

 appears, from Cohn's researches on Protococcus, Gonium, and other Phytozoa, 

 that they become released from their imprisonment, under the influence of 

 favourable external conditions, by the dehquescence of the rigid external sac, 

 and sometimes by its transfonnation into an external mucilaginous invest- 

 ment, and by the breaking-up of the internal protoplasmic cell into a number 

 of motile zoospores. 



The act of encysting may proceed with macrogonidia in their ' still ' con- 

 dition ; or it may overtake motile primordial cells, as in the case of EugJena 

 and of some phases of Protococcus, and in such, just as in the zoospores of 

 Algae, prove antecedent to further acts of development by fission. Cohn im- 

 plies, in his history of Protococcus, that microgonidia may themselves be en- 

 cysted ; and the same eminent observer describes the primordial cells of that 

 plant as in some instances suiToimding themselves with a fii'm external 

 envelope, pushing out two cilia, and moving about for a time in a 'swarming' 

 manner ere assuming the ' still ' condition, when the cilia disappear. But, 

 fiu'ther, he shows that gonidia, furnished with a rigid external waU, proceed 

 to develope others like themselves by self-division of their substance (XIX. 

 25), and that these secondary cells, each included within its own sac, go on 

 to divide into other spores, which, however, prove not to be ' still ' like their 

 parents, nor hke them encysted, but motile zoospores. 



In the aggregated family Yolvocinese, some or all the primordial cells be- 

 come encysted. When this takes place, their contents grow thicker, less 

 transparent, darker, and change from green to bro^\Ti and brownish, or to 

 a yelloA\ish red. At the same time, the interciUTent filaments disappear, the 

 cells themselves acquire a more s]3herical figm^e, and gradually loosen them- 

 selves from the common envelope, and move slowly about ^vithin it by means 

 of two ciha, until they at length escape by a rupture at some point (XIX. 44, 



