DEVELOPEMENT OF BATRACHIA. G23 



forming the intestinal canal, h, excludes a portion of the yolk, v : 



the tegumentary or 'serous' lay- 432 



er, a, accompanies the ' mucous ' 



layer, i, in the process of severing 



the vitelline from the intestinal 



cavities, and an outer yolk, or ' vi- 



tellicle,' results. 



The embryo of the Frog is ex- 

 tricated at a less advanced stage 

 of developement than that of any 

 other vertebrate animal: the neu- 

 ral lamina! have United along the Longitudinal section, Embryo of Fish, lxx-iv. 



trunk, and two of the haemal arches have become complete 

 below the head, but, in other parts, the neural and hremal canals 

 are closed only by the corresponding lamina? in a state of mem- 

 brane, the original investing membrane of the yolk being retained 

 over all. 



After extrication, the tadpole rapidly grows, and the chief 

 change of form is witnessed in the gills : each of the two lateral 

 gills puts forth four plates, which have vascular and richly ciliated 

 surfaces, fig. 430, c : a short additional leaflet is sometimes deve- 

 loped from the base of the hinder gill. ' The current of the 

 blood poured in regular pulsations at each contraction of the 

 heart passes up each stem or main branch of the branchia), and a 

 distinct stream is given off to each leaf; it is propelled to the 

 extremity, and then returns down the opposite sides in the most 

 regular manner, and the parts are so transparent that every 

 globule of blood is distinctly and beautifully visible.' 1 



The first cutaneous mouth is defined by epidermal jaws, in the 

 form of a very short transversely extended beak, fig. 433, 22, sur- 

 rounded by a lip armed with minute rasp-like denticles, and aided 

 by the pair of cephalic suckers projecting behind the mouth. The 

 wide pharynx, communicating also with the outer world by the 

 lateral branchial slits, is extended posteriorly by a short oesophagus 

 to a simple gastric enlargement, beyond which an equally simple 

 intestinal sac, laden with the remnant of the vitellus, gives issue 

 to a short and straight rectum, which is continued to the long 

 tegumentary and transitory cloacal canal at the fore-part of the 

 subcaudal fin. The contained yolk, fig. 431, hv, is not, as in Fishes, 

 fio\ 432, v, a mere 'food-yolk :' it is part of the germ-mass, and 

 consists of the embryonal cells, with their nutritious oil-globules. 

 Whilst, therefore, it serves to nourish the growing embryo, it also 



1 cccxvn. p. 101. 



