166 



THE EVOLUTION THEORY 



fishes and other freshwater aniinals, but that the drawback of the scanty 

 production of eggsJs_counteracted on the one hand by their Jiabit of 

 reproduciBg parthenogenetically f or ^thTgreateFpart oFffie j^ear.and 

 on the other hand by theirlTabrt.of concealing the eggs in a .special 

 brood-chaniber. This is the case not only in the sunnner eggs, to 



which nourishment is conveyed in the brood- 

 chamber from the blood of the mother (Fig. 

 70), but also in the wintei' or ' lasting ' eggs, 

 which recei\e within the chamber a pro- 

 tecting covering (the shell or ephippium). 



In almost all the Daphnids the winter egg 

 develops into a perfect animal just like that 

 to which the sunnner egg gives rise, although 

 it no longer recei\eH any nourishment after it 

 passes into the brood-chamber. But it receives 

 a larger supply of yolk on this account, so 

 that the nutritive provision within the egg is sufficient to develop the 

 perfect animal. There is only one exception to this, and it is of special 

 theoretical interest, because it shows more plainly than any other 

 fact that the greater or less degree of condensation in the ontogeny 

 depends upon the combined effect of the external conditions of life. 



Fig. 70 (repeated). Daph- 

 nella. .4, summer ovum, with 

 an oil-globule (Oe). B, winter 

 ovum. 



Flu. no. The largest of the Daphnids {Leptodora hyuUmi), with summer 

 ova {Ei) beneath the shell {Sch). 1-IX, the appendages. 11, the oars (second 

 antennte) which always remain biramose in Daphnids. sb, seta?, ov, ovaries. 

 Schl, oesophagus. Ma, stomach, a, anus. H, heart. Au, eye. nO, natural size. 



The largest of the Daphnidse, Leptodora hyalina, a beautifully 

 transparent inhabitant of our lakes, which measures about a centi- 

 metre in length (Fig. no), also emerges from the summer egg as a perfect 

 animal, but from the winter egg, which floats freely in the water and 

 ha.ji only a small provision of yolk, it emerges as a nauplius, which 



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