DEVELOPMENT OF RANA 183 



As in invertebrates, segmentation begins with the mesoderm 

 and extends to the other tissues. 



The Gut. — The gut is a cavity with an accumulation of yolk- 

 cells in the hinder part of its floor. This posterior region will 

 become the intestine, and in front of it will develop the pharynx, 

 oesophagus, and stomach. After the blastopore has closed, 

 the anus breaks through near the same spot, as a result of the 

 sinking in of an ectodermal pit (the proctodeum) till it meets 

 the endoderm, and perforation ensuing. In a similar way, the 

 mouth perforates in front, at the bottom of an ectodermal 

 pit (the stomodaeum). 



Behind the mouth, in the region of what will be the pharynx, 

 five pouches grow out on each side from the endoderm to the 

 ectoderm. These are the rudiments of the visceral clefts. 

 The first pair corresponds to the spiracles of the dogfish, but 

 here they do not become perforated to the exterior. Their 

 cavities persist as the Eustachian tubes. The remaining four 

 pairs of pouches become the gill-slits, through which the pharynx 

 communicates with the exterior. 



Alternating with the visceral clefts are the visceral arches. 

 The 1 st or mandibular arch separates the mouth from the 

 Eustachian tube (or hyomandibular cleft) ; the 2nd (or hyoid 

 arch) is between the latter and the 1st gill-slit. The 6th 

 visceral arch is behind the 4th gill-slit. 



From the upper part of the 3rd, 4th, and 5th visceral arches, 

 tufts grow out on each side which will become the external 

 gills ; blood-vessels enter them, and they serve as the first 

 respiratory organs. The dorsal part of the 1st gill-pouch on 

 each side proliferates to form a body which is the rudiment 

 of the thymus gland. 



In the floor of the pharynx between the 2nd gill-slits, a 

 downgrowth is formed, which ultimately loses its connexion 

 with the pharynx and forms the thyroid gland. Close to the 

 point of origin of the thyroid gland is an elevation which will 

 eventually give rise to the tongue. A little farther back, also 

 in the middle line of the floor of the pharynx, the rudiment of 

 the larynx appears as a groove. This deepens into a tube 

 remaining in connexion with the pharynx through the glottis. 



