DEVELOPMENT OF PHEYGANIDS. 553 



We have already seen that the gelatinous envelope swells on 

 contact with water, thus decreasing the specific gravity which 

 tends to hold the eggs more freely suspended in the water and 

 prevents them from being too deeply buried in the mud and 

 slime which would cut off the necessary supply of oxygen. 



It is possible that the gelatine may have some differential 

 effect upon the diffusion of gases held in the water^ excluding 

 some and hastening the transmission of others. It may also 

 serve as an attraction to Infusoria which feed upon the decaying 

 mass, and thus furnish a ready supply of food for the young 

 larvae. But it is certain that in the great variety of instances 

 where a similar covering is present in other animals which 

 deposit their eggs in the water, e. g. Molluscs, Amphibia, 

 Reptiles, &c., that it must have some other function than the 

 ones just mentioned. For in all the cases mentioned above 

 where we have pointed out a real or supposed advantage 

 accruing to one species or another, it is easily seen that to 

 other closely related forms, and were nearly the same con- 

 ditions were present, there would be no advantage whatever. No 

 one of these advantages is of such general importance in all 

 the different cases where we find the jelly present as to warrant 

 us in supposing that it was primarily present for that purpose, 

 or, at least, that it had its principal function therein. 



There is, however, still another function to which this en- 

 velope may be applied, and indeed it is one of far greater and 

 more general application than any as yet suggested. It is, 

 namely, that the gelatinous envelopes of certain eggs which 

 are deposited in the water have their chief function in protect- 

 ing the eggs against the parasitic growth of fungi and Infusoria. 

 I have observed a great many times that, when the gelatine 

 which envelopes the eggs of Bufo or Rana has been ruptured 

 and exposed the eggs to a more direct contact with the water, 

 they are very quickly covered and destroyed by fungi. It 

 might well be claimed that the same agent which ruptured 

 the envelope might have killed the embryos, and that of course 

 putrefaction and parasitic growths would follow. But this 

 objection can be easily met by the fact that not only the eggs 



