194 



HISTOLOGY. 



dermal part of the urogenital sinus; only the part toward the bladder, 

 which corresponds with the urethra in the female, is described as ento- 

 dermal. As already noted there is no line of demarcation between the 

 germ layers at this point, and a portion of the female urethra is by some 

 considered ectodermal. The bladder is to be described with the urinary 

 organs and the urethra with the genital organs. 



Returning to the intestinal portion of the entodermal tract, it is seen 

 that in early stages, A, the yolk sac extends from the pharynx nearly to the 



al 



FIG. 220. STAGES IN THE DEVELOPMENT OP THE DIGESTIVE TUBE. A, Rabbit of 9 days. B, Man 



2.15 mm. (after His). C, Pig, 12 mm. D, Man, 17.8 mm. (after Thyng). E, Man, about 5 months. 



a.. Anus; al., allantois; bl., bladder; cae., caecum; cl., cloaca; du., duodenum; 1. i., large intestine; oe., 



oesophagus; p., penis; ne., perineum; ph., pharynx; i., rectum; s. i., small intestine; St., stomach; 



u. c., umbilical cord; ur., urethra; ura., urachus; u. s., urogenital sinus; v. p., vermiform process; 



y. s., yolk sac ; y. St., yolk stalk. 



posterior limit of the entoderm. With further growth a posterior intestine 

 becomes formed by folding or constriction, comparable with the pharynx 

 in front (B). The connection between the yolk sac and the intestine be- 

 comes a slender yolk stalk, a part of which is shown in C and D. Later 

 it loses its continuity and the detached yolk sac remains until birth as a 

 small vesicle at the distal end of the umbilical cord, with which it will be 

 described later. The yolk stalk which extends from the umbilicus to the 

 intestine should be completely resorbed. It may persist as a fibrous cord 

 liable to produce intestinal obstruction, or the part near the intestine may 



