4 6 



REPTILES AND BATRACHIANS. 



dorsalis, a " neural tube,'' a heart, respiratory organs, and so forth, 

 "analogous to those of Vertebrates," until quite a respectable 

 little tadpole is developed (fig. 17, last page). 



In the frog's egg something of this kind may be watched from 

 day to day. The germ begins in the upper part. The illustrations 

 — much magnified, especially the earlier ones, as given in fig. 18 

 — enable us to trace the still further progress. I select them on 

 account of their distinctness, though tadpoles of a French, not an 

 English frog, but with a similar development. In the earliest (a) 

 we see that the embryo assumes a form before it leaves ■ the egg, 

 but there is no mouth, and no sign of an eye. Between TV and S is 

 a very slight depression, which is where the future mouth will be, 

 as you will observe by comparing the same spot with that in 

 fig. b and fig. c. In the two latter the tail is large and strong, 

 compressed laterally for swimming; no limbs appear at present. 

 At 5 the spot indicates a sucker, with which at first the young 

 tadpoles attach themselves to plants. At N is the nostril. In 

 fig. c, at Hz, the depression has developed into a bird-like 



Fig. 18. — a, Before it leaves the egg. 



" beak,'' or horny nippers, not the least like a frog's mouth, but 

 adapted to the little creature's wants, enabling it to nip the soft 

 water-plants which at this stage form its food, with doubtless the 

 minute forms of life attached thereto. Cope, the American 

 naturalist, tell us that the tadpoles of some frogs in the United 



