266 LECTUKES ON BIOLOGY 



If the nutrition becomes still lower so that it is not even able 

 to develop from the egg a male individual a large number of 

 primary egg-cells join together in order to form at the expense 

 of the whole mass one fertilizable winter egg." This explains 

 why at the beginning of the inclement weather, or before the 

 drying up of the home-water, suddenly the male daphnids appear 

 and ' winter eggs ' are formed. 



This result becomes still more clear -when we observe the 

 process of egg-forming in the water-fleas. Let us choose for 

 this purpose a well-known daphnid, Sida crystalline/,. The ovary 

 of this animal has the form of a long tube closed at one end. 

 At the blind end lies the germ-layer in which the young egg- 

 cells are formed, always four at a time, which are at first 

 equivalent, stored one behind the other, and filling the entire 

 lumen of the egg-tube. 



But the fate of these four young egg-cells is very different : 

 only one of them, no doubt the strongest, develops into a normal 

 ovum, while the other three are used as food. As new groups 

 of similar cells continue to proceed from the germinal layer, 

 there is a succession in the ovary at regular intervals of one 

 egg- cell with three food-cells. Afterwards egg-cells and food- 

 cells surround themselves with a yolk-membrane. With that 

 the ' summer egg ' is ready and may be voided into the brood- 

 cavity. In the ' winter eggs/ which are very rich in yolk, the 

 ovum is not satisfied with the ordinary food material, and thus 

 we observe that as many as twelve groups, that is, forty-eight 

 cells, are used for its construction. 



But the daphnids possess yet another arrangement for the 

 nutrition of their ' summer eggs.' The epithelial cover of the 

 ovary-tube consists in the adult female Daphnia, whose ovary 

 is filled with germ-cells, of cells so flat that they have been 

 overlooked by investigators until quite recent days. But in 

 the new-born females these epithelial cells are expanded so 

 much that they fill almost the entire lumen of the tube, and 

 make the whole ovary look like a solid chain of bladder-cells. 

 If now the young cells begin to advance from the germinal 

 layer they impress themselves deeply into these epithelial 

 vesicles which under the influence of pressure give the fluid 

 which they contain to the egg-cell and thereby resume their 



