74 AETHUR DENDY. 



the plug of cells is again removed, but this probably takes 

 place at Stage S, shortly before hatching. 



We may, perhaps, compare the plugging up of the nostrils in 

 the Tuatara with the remarkable solidification of the oesophagus 

 which takes place at a certain stage in the development of the 

 tadpole and other Vertebrates (Marshall [7] ).^ 



The Alimentary Canal. — The development of the ali- 

 mentary canal takes place in the usual way by gradual in- 

 folding of the splanchnopleure. The front part is enclosed 

 first (figs. 9, 10, 23, 24, 35), and for some time remains 

 filled with yolk particles (figs. 52, 59). The stomodaeum is 

 established in the ordinary way by a shallow invagination of the 

 superficial epiblast at Stages J and K (figs. 57, 63, 78). The 

 proctodaeal invagination is formed considerably later, com- 

 mencing at about Stage M. 



Of the outgrowths to which the alimentary canal gives rise 

 I have investigated only the allantois (which has been already 

 dealt with), the visceral clefts, and the commencement of the 

 pituitary body and of the thyroid gland. 



The visceral clefts are four in number, and appear to develop 

 very much in the usual way as outgrowths of the hypoblastic 

 lining of the front part of the alimentary canal (cf. figs. 63, 

 78, 91). The first becomes visible externally about Stage K 

 (figs. 74,75). At Stage L three are thus visible (figs. 83, 84). 

 At Stage N all four can be seen from the outside (fig. 94), and 

 at Stage O they begin to close up again (figs. 96, 98), though 

 the hyomandibular remains visible externally till Stage P 

 (fig. 101). 



The pituitary body was first observed at Stage L as a 

 thickening of the lining epithelium (presumably derived from 

 the stomodaeum) beneath the anterior extremity of the noto- 

 chord (fig. 90). At Stage N it forms a deep pit with a very 

 thick wall composed of columnar cells (fig. 95). 



The thyroid gland is visible at Stage K as a median^ventral 



1 Cf. de Menron, ' Compt. Rend.,' torn, cii, p. 1401 ; and ' Rec. Zool. Suisse,' 

 toni. iii. 



