FORMATION OF THE PRO NYMPH FROM THE LARVA. 309 



Although in the foregoing description of the histolysis of the 

 fat cells I have adopted the views of Kowalevski, it is by no 

 means certain that this interpretation of the phenomena 

 observed is the correct one. The fact that nests of leucocytes 

 first appear around the nuclei and in their interior lends pro- 

 bability to a view long ago propounded by Arnold that in 

 certain animal cells leucocytes are generated in large numbers 

 within the nucleus. It is very difficult to distinguish between 

 the emigration and immigration of leucocytes, and, although 

 the idea that leucocytes enter the fat cells and multiply within 

 them is perhaps more consistent with modern ideas, there is 

 much to be said in favour of the view that leucocytes are 

 formed within the nuclei and emigrate from them. Indeed, at 

 one time I felt convinced from the appearances which I 

 observed that this view must ultimately prevail, but at present 

 I regard the question as an open one. 



Relation of Fat to Proteids. From a physiological point of 

 view, the conversion of the cells of the fat bodies into nests 

 of leucocytes is of great interest. Whatever the nature of 

 the process may be, a large quantity of fat is converted into 

 proteid material, probably by direct combination with sub- 

 stances rich in nitrogen. This much is quite certain, that the 

 percentage of fat is very large in the mature larva, whilst little 

 remains in the newly-formed imago when it escapes from the 

 pupa-case. It is usually supposed that the fats are oxidized 

 during the process of transformation, but in summer, when 

 this is rapid, the loss of weight in the entire organism is very 

 small, certainly less than 5 per cent. ; hence the fat cannot be 

 accounted for in this way, and the rapid transformation of the 

 fat cells into nests of leucocytes points to its actual conversion 

 into proteid material. So long as we remain entirely ignorant 

 as to the nature of this fat, it is hopeless to attempt any solu- 

 tion of the problem, and the source of the nitrogen which 

 forms so large a proportion of the proteid is entirely unknown. 

 Possibly highly nitrogenous extractives co-exist in the fat bodies 

 with the fats which they contain. The subject would probably 

 repay investigation by a competent physiological chemist, as 



