194 RELATIONS OF OVA AND GEMMiE. 



remarks, to regard tlie germ cells and ova of the Aphides 

 as morphologically identical structures ; and the conclu- 

 sion is strongly corroborated by the researches of Mr. Lub- 

 bock into the process of reproduction in Entomostraca, and 

 particularly in the genus Daphnia. 



In Daphnia, Cypris, and probably others of this order, a 

 production of young has been observed to take place mth- 

 out impregnation — somewhat as in the case of the Aphides, 

 — ^and, as in them, it may be repeated for many generations 

 in succession ; only the young are not born alive, but 

 hatched from eggs. In these genera there are known to 

 occur two kinds of egg-like bodies, one capable of spon- 

 taneous development, and the other dependent on impreg- 

 nation. The eggs of the first kind are laid at frequent 

 intervals, and in large numbers, during the summer, and 

 very speedily develop their contents ; those of the other 

 kind are formed in sparing numbers at the close of the 

 season, and are termed winter eggs, as they serve the pur- 

 pose of continuing the race through the winter, being 

 defended by their dense envelopes against the cold, which 

 proves fatal to the parent animals, and not undergoing their 

 evolution till the following spring. In Daphnia, the winter 

 eggs are termed also ephippial, from their being carried for 

 some time in an ephippium, or saddle-shaped mass, nearly 

 in the same situation as the matrix for the ordinary ova, on 

 the back of the animal, under the shell, with which they are 

 thrown off in the process of moulting. Mr. Lubbock has 

 shown, that while the common sort of eggs in the Daphnia 

 are agamic — that is, capable of spontaneous evolution — the 

 ephippial, or winter eggs, probably require impregnation. 

 They differ also in their later development ; they are 

 then of larger size, they are enclosed in hard horny 

 bivalve shells, and they lie in a mass of dark coloured 

 pigment ; but both kinds of eggs seem, from the observa- 

 tions of Huxley, and of Lubbock, to be formed originally 



