AMPHIBIA. 



133 



..-a* 



long axis of the body, and the optic vesicles (a') are seen at its 

 sides. 



The rudiments of the mandibular (d), hyoid (<?), and first 

 branchial (/) arches project as folds at the side of the head, but 

 the visceral clefts are not yet open. Rudiments of the procto- 

 daeum and stomodaeum have appeared, but neither of them as 

 yet communicates with the mesenteron. Below the hyoid arch 

 is seen a peculiar disc () which is an embryonic suctorial organ, 

 formed of a plate of thickened epiblast. There is a pair of these 

 discs, one on each side, but only one 

 of them is shewn in the figure. At a 

 later period they meet each other in the 

 middle line, though they separate again 

 before their final atrophy. They are 

 found in the majority of the Anura, but 

 are absent according to Parker in the 

 Aglossa(PipaandDactylethra(fig.83)). 

 They are probably remnants of the 

 same primitive organs as the suctorial 

 disc of Lepidosteus. 



The embryo continues to grow in 

 length, while the tail becomes more 

 and more prominent, and becomes 

 bent round to the side owing to the 

 confinement of the larva within the 

 egg-membrane. At the front of the 

 head the olfactory pits become distinct. 

 The stomodaeum deepens, though still 

 remaining blind, and three fresh bran- 

 chial arches become formed ; the last 

 two being very imperfectly differenti- 

 ated, and not visible from the exterior. 

 There are thus six arches in all, viz. 

 the mandibular, the hyoid and four 

 branchial arches. Between the man- 

 dibular and the hyoid, and between 

 each of the following arches, pouches 

 of the mesenteron push their way 

 towards the external skin. Of these pouches there are five, there 



FIG. 79. TRANSVERSE SEC- 

 TION THROUGH A VERY YOUNG 

 TADPOLE OF BOMBINATOR AT 

 THE LEVEL OF THE ANTERIOR 

 END OF THE YOLK-SACK. (After 



Gbtte.) 



a. fold of epiblast continu- 

 ous with the dorsal fin ; is". 

 neural cord ; in. lateral muscle ; 

 as*, outer layer of muscle-plate; 

 j. lateral plate of mesoblast ; 

 b. mesentery ; . fold of the 

 peritoneal epithelium which 

 forms the segmental duct ; f. 

 alimentary tract ; f. ventral 

 diverticulum which becomes 

 the liver; e. junction of yolk- 

 cells and hypoblast-cells ; d. 

 yolk-cells. 



