REPKODUCTION BY GERM-CELLS 



283 



the extravasa,tion of the nutritive constituents of the blood into the 

 brood-cavity, and they thus require a smaller provision of yolk than 

 the winter eggs, which are thrown entirely upon their own resources. 

 Accordingly we find that in all Daphnids the summer eggs are at 

 least a little smaller and have less yolk than the winter eggs, as in 

 the genus Daphnella (Fig. 70, A and B), while in some species, e. g. of 

 Bythotre])hes, this difference increases so much that the summer eggs 

 are almost without yolk, and therefore very minute (Fig. 71, B). The 

 reason of this lies in the fact that in this case the brood-sac is filled 

 with a nutritive fluid rich in albuminoid substances, so that the 



Fig. 70. Daphnella. A, sum- 

 mer egg. B, winter egg. Oe, 

 ' oil-globules ' of the summer 



Fig. 71. Bijthotrephes longimanus. A, the brood- 

 sac (Br) of the female containing two winter-ova 

 {Wei), on which five large sperm-cells (sp) are lying. 

 B, dorsal surface of the animal. Br, glandular layer 

 which secretes the shell-substance. BK, copulatory 

 canal. B, The brood-sac (J5r) containing two summer- 

 ova {Sei). Both figures under the same magnifica- 

 tion (100). 



embryo during its development is continually supplied with con- 

 centrated nourishment. This is not the case with the winter eggs, 

 because these are liberated into the water, and we therefore find that 

 they are of enormous size and quite filled with yolk (Fig. 71, A). 



In this instance, as in all the simpler eggs, the yolk constituents 

 are secretions of the cell-body of the ovum ; but nature employs many 

 devices, if I may so speak, to bring up the mass of the egg, and 

 especially of the yolk, to the highest attainable point. Thus in many 

 orders of Crustaceans, for instance in the water-fleas just mentioned, 

 there are special egg-nourishing cells, that is, young ovum-cells which 



