RANA. 



357 



The step to the newly-hatched tadpole is not great. 



The exterior shows the long tail already formed, the developing eyes 

 and ears on the head, the two pair of external gills (soon followed by a 

 third), and, on the ventral surface of the head, a large sucker The 

 openings of the stomodcEiun and amis are also seen. The former is 

 blind, but the latter is already an aperture leading into the gut. In 

 longitudinal section we may notice the commencing liver as a ventral 

 diverticulum of the gut, and in front of it the simple tubular heart. 

 The pronephros is present as three ciliated funnels on each side, leading 

 by a paired archinephric duct on each side to the hind-end of the gut. 



^ig- 255. — Young Tadpole dissected from the Ventral 

 Side. 



(Mainly after Marshall.) 



Mouth. 



Afferent 

 Branchials. 



Opercular 

 Cavity. 



Heart. 



Pronephros. 



Pronephric Duct. 

 Aorta. 



•Opercular Cavity. 



Opercular Aperture. 

 Olomerulus. 



Mesonephros. 



Intestine. 



Hind-Limb. 



Cloacal 

 Aperture. 



A day or two after hatching the mouth opens,"with horny jaws ; the 

 yolk has been used up and the tadpole feeds upon small water-plants. 

 At the same time the four gill-clefts open and internal gills are formed 

 on their walls. The external gills then atrophy. The gill-slits be- 

 come covered over by a fold of skin or ope}citlum on each side. A 

 small opercular nperture remains on the left side, but none on the right. 

 The tail is provided with a dorsal and ventral median fin, and the tadpole 

 swims actively by its action. During this stage, which lasts for about 

 six weeks from hatching, the tadpole has an internal organisation like 

 a fish. The two-chambered heart drives blood by afferent branchials 

 to the four gills. The limbs develop as small buds, the fore-limbs in 

 the opercular chamber and the hind-limbs beside the anus. The 

 mesonephros arises as a set of small tubules which join the pronephric 

 duct and gradually replace the pronephros. The lungs then develop 

 and become functional. 



The tadpoles frequently come to the surface and take air into the 

 lungs. Thus is instituted a stage comparable to the Dipnoi, in which 

 both forms of breathing are functional. At about two months this 



