The Life-Cycle of Cladocera. 



423 



that the production of sexual forms can be provoked at will in any or every 

 generation of a particular species by alterations in the environment. 



It is important to consider whether the factor of crowding can ever 

 operate in a state of nature in the same way as under cultural conditions. 

 There can be no doubt that the way this factor exerts its effect is through 

 the presence of some excretory material in minute quantities, because in our 

 cultures of Daphnia, which were fed on nothing but Protococcus, it was 

 possible to ensure that there was in all cases an excess of food, so that the 

 crowding; could never cause a shortage of food. Since the animals were 

 cultivated in glasses containing about 100 c.c. water, and the presence of 

 10 individuals constituted the normal crowded condition, it is clear that 

 the reaction must be a very delicate one, due to the presence of extremely 

 minute proportions of the substance in question. Now, in a state of nature, 

 the small pools inhabited by many " polycyclic " species of Cladocera, e.g., 

 Moina, are often far more intensely crowded with individuals than under 

 our cultural conditions. But, quite apart from small pools, it is frequently 

 to be observed that large ponds are often so thickly populated with species 

 like D. magna and pulex as to be coloured blood red, and I have met with 

 cases where farmers have been afraid to water their horses at a pond on 

 account of the extraordinary colour of the water. I have also observed 

 that even in lakes, certain areas of water may be intensely crowded with 

 some species of Cladocera, and it appears to me probable that the factor 

 of crowding may play its part in the production of the sexual forms even 

 in the largest bodies of water. The interesting report of Dr. Viktor 

 Langhans on the Cladocera of the Hirschberg Lake in North Bohemia (4) 

 shows that the various species of Cladocera inhabit for the most part quite 

 localised areas of the lake, and, moreover, that the appearance of the sexual 

 forms usually either coincides with or follows closely after the greatest 

 activity in parthenogenetic reproduction, when crowding would be at its 

 height. 



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



in Cladocera. 



In the course of the breeding experiments described above, a contrast was 

 noticeable between the individuals isolated at 27° C. and those kept crowded 

 at a lower temperature. It was observed, even on inspection with the naked 

 eye, that the young or fully grown individuals isolated at 27° C. were always 

 of a pale, translucent green colour, while those crowded at the lower tempera- 

 ture were generally bright reddish orange, or, at any rate, showed a good 

 deal of this colour. On examining the two kinds of individuals under the 



