110 



GENERAL GROWTH. 



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 (Pipa and 

 Dactylethra (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 stomo- 

 da3iim deepens, though still remaining blind, and three fresh 

 branchial arches become formed ; the last two being very imperfectly 

 differentiated, and not visible from the exterior. There are thus six 



arches in all, viz. the mandibular, the 

 hyoid and four branchial arches. Between 

 the mandibular 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 being no pouch behind the 

 last branchial arch. The first of these will 

 form the hyomandibular cleft, the second 

 the hyobranchial, and the third, fourth and 

 fifth the three branchial clefts. 



Although the pouches of the throat 

 meet the external skin, an external open- 

 ing is not formed in them till after the 

 larva is hatched. Before this takes place 

 there grow, in the majority of forms, from 

 the outer side of the first and second 

 branchial arches small processes, each form- 

 ing the rudiment of an external gill ; a 

 similar rudiment is formed, either before 

 or after hatching, on the third arch; but the 

 fourth arch is without it (figs. 80 and 82). 

 These external gills, which differ fun- 

 damentally from the external gills of Elas- 

 mobranchii in being covered by epiblast, 

 soon elongate and form branched ciliated 

 processes floating freely in the medium 

 around the embryo (fig. 80). 



Before hatching the excretory system be- 

 gins to develop. Tlie segmental duct is 

 formed as a fold of the somatic wall at the 

 dorsal side of the body- cavity (fig. 79, ). Its 

 anterior end alone remains open to the body- 

 cavity, and gives rise to a pronephros with 



FIG. 79. TRANSVERSE SEC- 

 TION THROUGH A VERY YOUNG 

 TADPOLE OF BoMBINATOR AT THE 

 LEVEL OF THE ANTERIOR END OF 



THE YOLK-SACK. (After Gotte.) 



a. fold of epiblast continu- 

 ous with the dorsal fin; ?V. 

 neural cord ; m. lateral muscle ; 

 as*, outer layer of muscle-plate ; 

 s. lateral plate of mesoblast; 

 b. mesentery; ?/. fold of the 

 peritoneal epithelium which 

 forms the segrueiital duct ; /. 

 alimentary tract; /'. ventral 

 diverticulum which becomes 

 the liver ; e. junction of yolk- 

 cells and hypoblast-cells ; d. 

 volk-cells. 



