896 GENERAL METABOLISM [pt. iii 



clusively demonstrated by Ramult for Daphnia, Ceriodaphnia, Scaphole- 

 beris, and Simocephalus . 



Another case in which it has been shown that the tgg contains 

 enough solid but not enough water for the finished embryo is that 

 of the ovoviviparous batoid elasmobranch, Torpedo marmorata. Davy 

 found as long ago as 1834 that the mean weight of the egg when 

 undeveloped was 182 grains, that of the egg plus early embryo was 

 1 77 grains, while that of the finished embryo was 479 grains. Allowing 

 80 per cent, of water for the mature state, which is very reasonable, 

 only 95 grains would be required of non-combusted solid, so that 

 it is very probable the increase was all due to water, especially as 

 the gelatine-Hke egg-cases would hardly allow anything. but water 

 to pass through them, Vidakovich afterwards found an increase in 

 weight of the finished embryo over the egg, of 40 per cent., and from 

 my own observations of the fish on which he worked, Squalus acanthias, 

 I believe that the events which take place there are quite analogous 

 to those in the trout, thus; 



Parker & Liversidge, again, reported that the undeveloped eggs of 

 Mustelus antarticus, an ovoviviparous selachian, measured 43 x 16 

 X 10 mm. (roughly) while the ripe embryos ready to hatch measured 

 220 X 25 X 25 mm. 



A very remarkable case is that of the (Siluroid) catfishes which 

 incubate their embryos in their mouths. Wyman in 1857 studied 

 several species of the genus Bagrus at Paramaribo in Dutch Guiana, 

 and found that the hatched embryos not yet liberated from the 

 parental mouth weighed considerably more than the undeveloped 

 eggs. His conclusion that a nutritive fluid was supplied does not of 

 necessity follow ; it is likely that a good deal of water was absorbed. 



Again, in gastropods the eggs swell greatly, according to Nekrassov. 

 As regards Crustacea Needham & Needham, in the course of other 

 work on the chemical embryology of the sand-crab, Emerita analoga, 

 observed that the water-content of the whole egg was 63 per cent, in 

 the cleavage stages, 75 per cent, about the middle of development, 

 and 85 per cent, at the time of hatching. But the extreme case is 

 undoubtedly the desiccation that phyllopod (e.g. Artemia salina) and 

 cladoceran eggs may go through before their development. Wolf found 



