402 RICHAED EVANS. 



structure of the nuclei of the flagellated cells is a certain 

 amount of contraction^ which in itself suffices to account to 

 some extent for the density of structure found in them during 

 these stages, though by no means sufficient to explain the whole 

 of it. Another reason is probably to be found in the fact that 

 when the flagellated cells pass to the interior they come within 

 reach of a quantity of food material stored up in the cells with 

 vesicular nuclei, and available for their use. When at the 

 surface they are to all intents and purposes starved, the result 

 being the arrest in the progress of their development that has 

 already been mentioned when describing the larva. In conse- 

 quence of this partial starvation, the flagellated cells, on passing 

 to the interior, are attracted to the stores of food material 

 which they find there. The nucleus being the main instrument, 

 if not the only instrument of constructive metabolism, is pro- 

 {)ortionately affected and altered in structure ; while the cell 

 body itself, which was never very big and is not destined to 

 grow to a very great extent, becomes plastered to the body 

 of the cell in which the food material is stored up. Hence 

 the changes which go on in the flagellated cells after emigra- 

 tion seem to result from absorption of food material and con- 

 sequent increase of nucleoplasm, especially of the chromatic 

 portion of the nucleus. 



Simultaneously with the contraction of the nucleus de- 

 scribed above the nuclear threads become thicker, and the 

 chromatin rearranges itself. Instead of being scattered about 

 in small granules, it appears, as a rule, as irregular patches 

 lying against the nuclear membrane, though at this stage it 

 does not invariably conform to a definite type of arrangement. 

 At the commencement of the above process of adhesion the 

 outlines of the flagellated cells are visible (fig. 29 h) ; but 

 later on they become so closely adherent that they are indis- 

 tinguishable as separate units in the morphological sense. As 

 a rule they fix themselves to the cells with vesicular 

 nuclei, but it often happens that a number of them become 

 attached to cells with granular nuclei, which in some 

 cases contain several yolk bodies, and which therefore act as 



