462 AMPHIBIA. 



At this stage we may briefly summarise the history of the germinal 

 layers. 

 The epiblast or ectoderm gives origin to the followmg structures :— 

 The central nervous system and its outgrowths, the nerves, the optic 



cups, the pineal or parietal eye, part of the hypophysis ; 

 The epidermis and its ingrowths, which form the lens, the ear-sac, 

 the nasal sac, the stomatoda2um. 

 The hypoblast or endoderm gives origin to the following structures :— 

 The lining of the mid-gut, and of its outgrowths, such as giU-clefts, 



lungs, liver, pancreas, and allantoic bladder ; 

 The notochord. 

 The mesoblast or mesoderm gives origin to the followmg structures :— 

 The dermis or cutis, the muscles, the skeleton, the vascular system, 

 besides aiding in the formation of organs, such as eyes and lungs, 

 the essential parts of which are due to the other layers. 



We may divide the development of the frog into three 

 periods; the first ends with the hatching, the second includes 

 the time between the hatching and the final metamorphosis, 

 the third is the period of metamorphosis when the character- 

 istic features of the adult are attained. We have summarised 

 the first chapter of the history, let us now turn to the 

 second. 



The young tadpoles are still without an open mouth. 

 They live on the still unexhausted capital of yolk which has 

 been absorbed into the gut. Behind the indimpling which 

 marks the incipient mouth, is a paired sucker, by which, when 

 tired, the tapdoles fix themselves to water-weeds. In a few 

 days, however, they gain a mouth, which is "bordered by a 

 pair of horny jaws, and fringed with fleshy lips provided with 

 horny papillae." The whole arrangement reminds us of the 

 lamprey's mouth. 



Milnes Marshall and E. J. Bles have observed the strange 

 fact that for a period commencing shortly before the opening 

 of the mouth, and lasting some little time after the opening 

 is effected, the oesophagus is solid. It is also remarkable, as 

 these investigators point out, that the tadpole undergoes a 

 distinct increase in bulk prior to the opening of the mouth, 

 in explanation of which it is suggested that nutriment, such as 

 the jelly of the spawn, may be absorbed by the cells of the 

 sucker. 



Having acquired a mouth, the tadpoles hungrily feed on 

 water-weeds ; the hitherto short alimentary canal becomes 

 elongated and coiled like a watch-spring. About the time 



