THE ALIMENTARY CANAL. 



147 



sharply marked off from the pharyngeal region by a backwardly 

 directed diverticulum (Fig. 60, l), which forms the first com- 

 mencement of the liver ; while at the hinder end of the body, by 

 withdrawal of the yolk-plug from the surface of the embryo (cf. 

 Figs. 55 and 60), the posterior limit of the yolk-mass becomes 

 clearly defined, and the short rectal diverticulum (Fig. 60, k) 

 opened out. 



At the time of hatching of the tadpole (Figs. 69 and 74), 

 this distinction between a wide, thin-walled pharyngeal region, 



Fig. 70. — Transverse section across the middle of the length of a Frog 

 embryo 3| mm. in length. (Cf. Figs. 58, D, and 60 for other views of 

 embryos of the same age.) x 52. 



CH, notocliord. C J. subnotocliordal rod. CM. myocal. CS, splanchnocoel. E' 

 epiblast. KB, archinephric duct. M, mesoblast. MS, mesoblastic somite. WD, dorsal 

 root of spinal nerve. NS, spinal cord. SO, somatopleuric layer of mesoblast. SP, 

 splanchnopleuric layer of mesoblast T, intestinal region of 'mesenteron. Y, yolk- 

 cells. 



and a narrow, thick-walled intestinal portion is very well 

 marked, the passage from one region to the other (Fig. 74) 

 being an abrupt one. Up to this time the alimentary canal has 

 been perfectly straight, but shortly after hatching, and especially 

 after the formation of the mouth, the intestinal region elongates 

 very rapidly ; the food-yolk is speedily absorbed, and the intestine 

 becomes a long tube, coiled in a characteristic spiral manner, and 



