55 



the primitive nucleolus diminishes in volume little by little, 

 finally disappearing. The number of nucleoli varies at every 

 instant. Some disappear whilst others are forming ; they 

 commence in the form of a minute point scarcely perceptible. 

 This point grows to a certain limit ; it becomes a veritable 

 corpuscle, formed of a homogeneous, highly refractive sub- 

 stance, then the corpuscle diminishes in volume ; it refracts 

 the light less and less ; finally it disappears. It even happens 

 that all trace of nucleolus disappears in the nucleus, and some 

 instants later one or several nucleoli can be distinguished 

 there, which undergo afresh all the variations which I have 

 just described. This fact of the successive apparition and 

 disappearance of the nucleoli in a nucleus of a cell, and the 

 modifications which occur in the nucleus as to number, dimen- 

 sion, and character of the nucleoli, has never yet been pointed 

 out to my knowledge. It appears to me to have a great im- 

 portance in relation to the cell theory. The idea of the 

 existence of a membrane round these little bodies (the 

 nucleoli) , and of their vesicular nature, is by no means recon- 

 cilable with the rapidity of their formation, and the modifica- 

 tion which they undergo in the course of some minutes ; and 

 if it is demonstrated that the membrane is not an essential 

 part of the cell, and that the nucleolus is sometimes, if not 

 always, devoid of membrane, may it not be presumed that 

 the nucleus of a cell is not necessarily a vesicle, and that, 

 contrary to the generally received opinion,^ a nucleus of a 

 cell may be equally devoid of membrane ? 



The Gregarinse move, and three kinds of movements may 

 be distinguished in them : 



1st. They present a very slow movement of translation, in 

 a straight line, and without the possibility of distinguishing 

 any contraction of the walls of the body which could be con- 

 sidered as the cause of the movement. Further, it is very 

 difiicult to account to oneself for the cause of this movement 

 of translation, at least to admit — what is difficult to demon- 

 strate — that the Gregarina acquires an adherence to the 

 surface on which it moves. Invariably the appearance of 

 this movement recalls completely that of the Turbellaria ; but 

 it is impossible to distinguish the least trace of vibratile cilia 

 on the surface of the body of these animals, even Avith the 

 highest magnifying powers. 



2nd. The Gregarina of the Lobster presents another kind 

 of movement, consisting in the lateral displacement of every 

 part, taking place suddenly, and often very violently, from a 

 more or less considerable part of its body. Thus the posterior 

 part of the body may be often seen to throw itself out laterally 



