C22 



ANATOMY OF VERTEBRATES. 



Longitudinal section, Embryo of Frog, lxxiv. 



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- 

 431 sents a semioval section, 



h v : it is inclosed, as in 

 fig. 428, by the haemal 

 prolongations of the or- 

 ganic layer forming the 

 abdominal parietes, «, 

 and lined by the ' mu- 

 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 Batracliia. 

 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, Husconi observed 

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

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

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

 four weeks. In Alytes obsteti'icans, the developement of the 

 * 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, hv, 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, i, in 



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



