THE MIDGUT 



205 



yolk. The oesophageal plug disappears by the 9 to 10 mm. stage of 

 development. 



Beginning at about the 1 1 mm. stage, the oesophagus develops into 

 the very distensible organ of the adult while the stomach becomes 

 differentiated simply by a dilation of the next adjacent part of the 

 foregut. By the time of metamorphosis the stomach is somewhat more 

 distended than the oesophagus but it is otherwise indistinguishable 



Rectum 



Bladder 



Cloaca 

 Anus 



Digestive system and derivatives. (A) Before 

 metamorphosis. (B) After metamorphosis, 

 (Modified and redrawn from the Leuckart 

 wall chart.) 



from it. In the newly metamorphosed frog, however, it assumes a 

 transverse position, shifting from the earlier longitudinal position, 

 and has all the characteristic layers of any vertebrate gut. This in- 

 cludes an inner and glandular layer of mucosa, intermediate layers of 

 connective and muscular tissue, and an outer covering of serosa. 



The Midgut 



The midgut is that portion of the original archenteron which is 

 found dorsal to the yolk mass as long as this mass exists, having a 

 roof and sides one cell in thickness. The floor is the thick yolk endo- 

 derm. After the time of hatching the yolk is rapidly absorbed. Be- 

 fore the time of metamorphosis the heretofore undifferentiated 



