264 i>l{OF. ALLMAN O'S THE KECEKT RESEARCHES 



Hertwig and Lesser have paid especial attention to the struc- 

 ture of the nucleus, and have shown that in it and its contained 

 nucleolus there is a remarkable constancy of character. The 

 nucleus presents the appearance of a clear vesicle whose contents 

 are either sparingly or not ab all coagulable by acetic acid : whiie 

 the nucleolus which it encloses is either a simple oval pale bluish 

 body, or appears to have been broken up into several such bodies ; 

 it becomes granular in weak acetic acid, and in stronger acid 

 swells up without becoming dissolved. In -some cases the nucleus 

 is seen to be bounded by a delicate structureless membrane-like 

 layer (nucleus membrane), though in others no definite boundary 

 layer can be demonstrated. 



The same authors have distinguished in these organisms two 

 kinds of locomotion. In one the contractility of the protoplasm 

 afi"ects equally the whole mass ; the body changes but slightly its 

 contour, and glides over the supporting object by a constant rota- 

 tion of its whole surface, as is seen in Hyalodiscus, Hert. & Les. 

 In the other, which is by far the more frequent condition, locomo- 

 tion is eifected by the contractility of limited portions of the sur- 

 face, either causing the protrusion and retraction of blunt or 

 pointed pseudopodia by means of which the organism is pushed 

 or drawn forwards, or giving rise to a streaming forth of the pro- 

 toplasm by which the whole body seems, as it were, to flow forward 

 in a definite direction. 



There is no definite orifice for the ingestion of nutriment, which 

 gains access to the interior of the body solely by transmission 

 through the surface of any part of the protoplasm which may be 

 exposed to the surrounding medium. Solid nutritious matter 

 thus becomes pressed into the deeper parts of the body, where 

 during assimilation it may generally be seen accumulated in 

 pellets surrounded by a clear liquid and included in a simple 

 vacuole, from which the effete residue becomes afterwards ex- 

 pelled, and is finally ejected through any part of the exposed 

 surface of the protoplasm. The attempt to confine the process 

 of as&imilation to the endosarc and of contractility to the ectosarc 

 is not supported by careful observation. Indeed the absence of 

 specialization in this most generalized phase of nutrition is further 

 apparent from the fact that the whole process may take place even 

 in a pseudopodium. 



In almost every case, as already said, vacuoles occur distributed 

 through the protoplasm. These are filled with a clear liquid, and 

 are either variable in number and indefinite in position, or they 



