No. 523] REPRODUCTION OF DAPHNIA 411 



ually acquired. The Malpighian layer of the skin may 

 be stimulated to proliferate more rapidly, but if the 

 stimuli are too strong the growth will be retarded instead 

 of increased. On gradually increasing the stimulus im- 

 munity to it may be acquired. 



To apply this hypothesis to the daphnids: conditions 

 which are adverse to the growth of the body cells, such 

 as extremes of temperature (viz., high temperatures) or 

 of concentration of excretory products, or disorded nu- 

 trition, either fail to retard the development of the germ 

 cells or stimulate their development, so that in either 

 case the daphnid becomes sexually mature at a less de- 

 veloped stage. Under the less extreme conditions the 

 eggs develop on receiving the slight stimulus incident to 

 their transfer to the brood pouch, but under the more 

 extreme conditions those eggs which develop at all must 

 be stimulated by fertilization before they develop. 

 These two types of eggs may perhaps develop from two 

 kinds of cells, or the sexual egg may arise from the same 

 kind of cell producing the parthenogerietic egg by ac- 

 quiring an immunity to slight stimuli. Whereas more 

 than one cell goes to make up a single egg; only one 

 nucleus is retained, and it may be said that one cell is the 

 egg cell and the remainder furnish its food. 



