622 



ANATOMY OF VERTEBRATES. 



431 



obtuse as the head, the more acute and longer free part as the 

 tail. In this growth the amphibian passes from a state in which 

 a longitudinal section would show it supported by a spherical 

 yolk, to that represented in fig. 431, in which the vitelline, or 



( haemal,' portion pre- 

 sents a semioval section, 

 h v : it is inclosed, as in 

 fig. 428, by the ha3inal 

 prolongations of the or- 

 ganic layer forming the 

 abdominal parietes, a, 

 and lined by the f mu- 



Longituclinal section, Embryo of Frog. LXXIV. > i -i 



cous layer, i : this be- 

 comes differentiated as the tunics of the alimentary canal, inclosing 

 the vitellus as the primary contents of such canal in all Batrachia. 

 The canal now communicates with the bucco-branchial cavity ; 

 and this opens externally on the lower part of the head by a 

 vertical fissure, on each side of which a small protuberance buds 

 out, forming a special organ of adhesion a pair of temporary 

 cephalic limbs. A pair of branchiae budding out from the gill- 

 aperture, the whole yolk being now closed in by both the in- 

 testinal and cutaneous layers, and the tail having gained its 

 muscular segments and cutaneous border-fin, the little tadpole, 

 by increasing vigour of its movements, bursts the egg-mem- 

 branes and comes forth. The external stimulus which most 

 influences this stage is warmth. In Italy, Rusconi observed 

 the eggs of the Frog to be hatched in four days ; Bauer 

 figures one extricating itself, in a warm spring, at Kew, after 

 the fifth day : * in a cold spring, it may be prolonged through 

 four weeks. In Alytes obstetricans, the developement of the 

 f mucous' layer proceeds to form a convoluted intestinal canal 

 before ( extrication.' In Rana esculenta, and probably other 

 Frogs, the vegetative organs are later in developement, and the 

 cavity, fig. 431, h v, has not assumed the intestinal form when 

 the embryo quits the egg : but in all Batrachia the whole yolk is 

 wanted for the formation of their long spirally wound larval gut. 

 Herein is a differential character between the Batrachian and the 

 Fish. In the latter, the supply for the mid-period of develope- 

 ment is received, primarily, from the vascular rather than from 

 the digestive system, and a part only of the yolk is required for 

 the formation of the straight and simple intestinal canal. Ac- 

 cordingly, the mucous layer, as in the diagram, fig. 432, ?', in 



1 cccxvi. pi. vi., fig. 1 A. 



