Reproduction 131 



for providing enough ash within the egg is acquired. To the fac- 

 tors defined by SoUas and von Martens we must add the importance 

 of the fresh-water egg being supplied with materials that its inor- 

 ganic surroundings cannot be expected to give it" (Needham, 

 1930) . "The ova of Nereis are capable of fertilization and devel- 

 opment in thirty-three and one-third per cent sea water" (Just, 

 1930, 1930a), but those of Echinarachnius and other marine ani- 

 mals die in rather slight dilutions. An eel, after a prolonged so- 

 journ in fresh water, is prepared to reenter the ocean and lay its 

 eggs by the increase of osmotic pressure in its body fluids, and this 

 apparently occurs without any stimulating change in its surround- 

 ings (Johnstone, 1908) . Gastrotricha show an interesting reaction 

 to salinity. In the ocean they produce an abundance of males and 

 zygotes, but in fresh water reproduction is parthenogenetic (Re- 

 mane, 1929). 



As some animals are limited to life in the ocean by salinity re- 

 quirements, so some are kept in aquatic habitats by inability to carry 

 on reproductive activities in the absence of water. The eggs of 

 most fishes and amphibians are laid in water, but a few deposit them 

 in air so that hatching young fall into the water. The spotted sala- 

 mander, Ambystomd maculatum (Shaw), lays its eggs in ponds, 

 but spends most of its adult life on land. Blanchard (1930) has 

 studied the breeding habits of this species. "Migration to the 

 breeding ponds depends on rain and not on temperature ... in 

 southern Michigan the spotted salamanders begin migration to the 

 breeding ponds during the first rain at night following the disap- 

 pearance of snow and thawing of the surface of the ground in the 

 woods; ... a prolonged rain, or several rains, will be required to 

 bring all adult individuals of the species to the breeding sites." The 

 marbled salamander, Amby stoma opacum (Gravenhorst) , drowns 

 in water. Its eggs are enclosed in four capsules and hence are fitted 

 to some degree for life on land. They are usually deposited in situa- 

 tions where hatching tadpoles will be washed into water by rains; 



