The Life-Cycle of Cladocera. 



429 



to rapid growth and to continuous parthenogenetic reproduction, which is to 

 he looked upon as a mode of growth by budding. The condition of crowding 

 and low temperature, on the other hand, stimulates the storage of fat as 

 opposed to glycogen, and this storage of fat tends to inhibit growth and to 

 call forth the production of the sexual forms of male and female, which are 

 pre-eminently characterised by abundance of fat-storage and retarded growth 

 and reproduction. Stated in a short and summary fashion, it is claimed 

 that conditions which favour glycogen metabolism lead to rapid growth and 

 parthenogenesis, while conditions which favour fat-metabolism lead to 

 inhibition of growth and the production of sexual forms. 



The way in which the factor of crowding leads to fat-storage, inhibition of 

 growth, and the production of sexual forms is still somewhat obscure. But 

 it is clear that the crowding does not act through partial starvation, because 

 in all cases there was an excess of the food material present upon which the 

 Daphnia were known to be feeding. This was ensured by feeding the 

 animals on a pure culture of green Protococcus, which constituted the sole 

 food of the organisms. The only other way in which crowding can be 

 conceived to exert an effect is by the accumulation of some excretory 

 product in the water as the result of the presence of numerous individuals. 

 It is reasonable to suppose that this excretory matter might act in some- 

 thing the same way as phosphorus on a warm-blooded animal, namely, by 

 stimulating the production of fat. All attempts at isolating or collecting 

 this supposititious excretory matter have hitherto failed, and it would appear 

 that it is easily destroyed, possibly by oxidation or bacterial action. 



3. The Storage of Fat and Glycogen in its Relation to Growth and Reproduction 



in Decapod Crustacea. 

 We may now consider how far the theory of the connection between reserve- 

 storage and growth and reproduction in the Cladocera harmonises with what 

 we know of these processes in the higher Crustacea. Ever since the writings 

 of Claude Bernard and the more recent work of Vitzou (2), it has been known 

 that the growth and moulting of the higher Crustacea is accompanied by a 

 remarkable heaping up of glycogen in the liver and subdermal connective 

 tissue. If we take sections through the liver of a crab, such as Carcinus 

 mamas, which is about to cast its skin in the course of a day or two, it will be 

 found, by staining the sections with iodine or neutral red, that the liver cells 

 are crammed with small round granules of glycogen, to the exclusion of almost 

 any other material (fig. 9). At this period there is practically no fat and the 

 protoplasmic content of the cells is small. Besides these storage cells of the 

 liver, the ferment cells, with darker protoplasm and larger nuclei, will be 



