]26 DEVELOPMENT OF THE FROG 



time after hatching, but shortly before the opening of the 

 mouth. About the time that the lungs first appear, in tadpoles 

 of about 8 mm. length, the oesophagus, which up to this time 

 has been tubular, becomes solid, and remains so until a short 

 time after the formation of the mouth. The meaning of this 

 curious point has not been ascertained. 



2. The stomodaeum. At the stage represented in Fig. 27, 

 shortly after closure of the neural canal, a conical ingrowth, p, 

 of the nervous layer of the epidermis is formed at the anterior 

 end of the body immediately below the fore-brain : from this 

 ingrowth the pituitary body is developed, and a slight depres- 

 sion of the surface epiblast opposite its base, marks the position 

 of the stomodfeum. 



At the time of hatching, this depression is a small shallow 

 pit, separated from the anterior end of the mesenteron by a thin 

 septum. Soon after hatching, in tadpoles of about 9 mm. 

 length, this septum becomes perforated, and the alimentary 

 canal commimicates with the exterior through the stomodaeal 

 pit. After the perforation is effected, the lips with the whole 

 anterior part of the face grow forwards rapidly ; the homy jaws 

 are formed, and the tadpole begins to feed vigorously. (Cf. Figs. 

 28 and 29.) 



The pituitary body (Figs. 27 to 29, P) is formed from the 

 ingrowing stalk of epiblast described above : this rapidly 

 elongates, growing backwards between the brain and the roof 

 of the mesenteron until it reaches the inf undibulmn ; its hinder 

 end now becomes tubular, gives ofi" a few lateral diverticula ; 

 separates from the stalk, which soon disappears, and becomes 

 applied to the ventral surface of the hinder end of the infundi- 

 bulum to form the pituitary body. 



3. The proctodaeuiu or anal invagination appears before the 

 stomodaeum. Shortly before the neural folds have met to 

 form the neural tube, the proctodseum is visible as a small 

 median depression of the epiblast at the hinder end of the 

 embryo, a little way below the blastopore. The cells lining it 

 are rather strongly pigmented, and slightly larger than the 

 surrounding epiblast cells. 



From the hinder end of the mesenteron a rectal diverticulum 

 (Fig. 27, R) extends downwards towards the proctodseum; a 

 little later, and some time before the tadpole hatches, the two 

 structures meet ; perforation occurs ; and the definitive anal or 



