602 



THE DIGESTIVE SYSTEM 



MESONEPMRIC 



/ J<^' ' "'"'"'''Sk. 7\ OIVERTICULUM-..^jQ^ 



MIDGUT J- ^ "^V \ /n\ 



/ L' , 'V Y,OIVERTICULUM ( / 



/ f> ,' '-^ *5&i °^ HINDGUT I / 



\ V.'' ' -ra / VENTRAL / , 



SEE 4LS0 FIGURE 296 



aOULT FORM ^ 



Fig. 280. Morphogenesis of the digestive tract in the frog. Rami pipiens. (See Chap. 10.) 



may be large and pouch-like, as in certain mammals, or slender and elongated, 

 as in birds. 



e. Cloacal and Proctodaeal Area 



The most prominent cloacal diverticula occur ventrally. Ventral urinary 

 bladders arise in this area in many vertebrates. The allantoic diverticulum 

 (Chap. 22) is a prominent outgrowth of the ventral wall of the cloaca. In 

 the chick, the bursa of Fabricius projects dorsally from the area between the 

 cloaca proper and the proctodaeum. Dorsal urinary bladders occur in fishes, 

 arising as dorsal diverticula within this general area. The anal glands of 

 certain mammals, such as the dog, represent proctodaeal evaginations. 



B. Development of the Digestive Tube or Metenteron 



The following descriptions pertain mainly to the developing shark, frog, 

 chick, and human embryos. Other forms are mentioned incidentally to empha- 

 size certain aspects of digestive-tube development. 



1. General Morphogenesis of the Digestive Tube 



The general morphological changes of the developing digestive tubes of the 

 shark, frog, chick, and human are shown in figures 279-282. 



