REPRODUCTION BY GERM-CELLS o 83 



the extravasation 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 res,,,,,-,. 

 Accordingly we find that in all Daphnids the summer egg 8 are 

 least a little smaller and have less yolk than the winter e as i„ 



the : genus Daphnella (Fig. 7 o, A and B), while in so, pecies , ,i 



Bythotrepkes, this difference increases so much that the summer e 

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

 reason of this lies in the fact that in this case the brood-sae is Blled 

 with a nutritive fluid rich in albuminoid substanc , thai the 



Br 



5« 





MSMm 



s P 



Fig. 70. Daphnella. A, sum- 

 mer egg. B, winter egg. Oe, 

 • oil-globules ' of the summer 



A 



WMKES 



egg. 



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

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

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

 R, dorsal surface of the animal. Br, glandular 1. 

 which secretes the shell-substance. BK, copulatory 

 • canal. B, The brood-sac (Br) containing two Bommer- 

 ova (Set). 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 i __■-. 

 because these are liberated into the water, and we therefore find tli.it 

 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 



