3IO 



Veterhiary Obstetrics 



creases in length far more rapidly than the body and drops away 

 from the dorsal portion of the body cavity to float freely, con- 

 fined in position only by the mesentery, which it has derived 

 from the superior wall of the abdomen and carried with it in its 

 descent. 



At first the posterior gut of the embryo represents jointly the 

 intestine and the genito-urinary passages and constitutes a sin- 

 gle dilated chamber, or cloaca, but later there grows back, from 

 the angle between the stalk of the allantois and the gut, a parti- 

 tion which serves to separate the digestive tube from the genito- 

 urinary tract. This partition is sometimes incomplete, especially 

 in the female when the proctodeal opening fails in the upper 

 portion and causes a closed anus, when the feces from the 

 intestine drop into the vagina, to be expelled through the vulva, 

 as indicated in Fig. 42. 



Fig. 42. Atresia Ani. Lamb. 



R, Rectum. P, Closed proctodeal pit. C, Cloaca. 

 Ur, Urethra. U, Uterus. B, Bladder. V, Vagina. 



The IvUngs. 



The lungs are formed as an outgrowth from the floor of the 

 pharynx in the region of the first to third branchial arches and 

 begin as a longitudinal groove, which soon develops into a blind 

 pouch. This evagination extends backward beneath the pharynx 

 and at its distal end soon splits into right and left halves, to con- 

 stitute the two lobes of the lungs. The growth continues back- 



