68 



SEX AND HEREDITY 



would at once classify it with the fishes. It is in fact a 

 Fish stage in development. 



It will readily be understood what a serious difficulty 

 the existence of such aquatic fish-like stages of develop- 

 ment has constituted in the w r ay of the assumption of a 

 purely terrestrial habit. The present lecture will deal 



with the methods by which the group 

 of animals to which we ourselves 

 belong the Vertebrates have over- 

 come this particular difficulty. 



\Ye may commence with the Am- 

 phibians the group of Vertebrates 

 which includes Frogs and Toads a 

 group which is of special interest from 

 the fact that it has not succeeded in 

 emancipating itself entirely from the 

 ancestral watery environment, but 

 yet has made a number of very 

 interesting attempts in this direction. 

 In the early spring one may see in 

 ponds and ditches masses of spawn 

 of the ordinary frog the eggs or 

 zygotes having the appearance, as 

 seen from above, of little black 

 spheres about -^V inch in diameter, each enclosed in a 

 larger sphere of clear crystal jelly. In due course the 

 eggs develop into tadpoles, which represent the fish stage 

 of development. In the case then of the ordinary frog 

 or toad, although the adult has emancipated itself from 

 the water to a certain extent it is able to live on land 

 though it needs a moist atmosphere the early stages of 

 its life-history are still purely aquatic. 



In an interesting Tree-frog called Phyllomednsa, which 



FIG. 37. 



Phyllomedusa sauvagii, mass 

 of spawn enclosed between 

 leaves. (From Graham Kerr's 

 Embryology, after Agar.) 



