SUBMAXILLARY GLAND 



245 



Serous Intercalated Blood 

 cells. duct. vessels. 



Secretory 

 duct 



Connective tissu 



Mucous cells. 



w. Fat cells. 



PIG. 240. SECTION OF THE SUBMAXILLARY GLAND FROM A MAN OF TWENTY-THREE YEARS. X 100. 

 Note that the serous cells predominate, and that secretory ducts are abundant. (A characteristic 



crescent is shown at z.) 



THE DEVELOPMENT OF THE DIGESTIVE TUBE. 



The digestive tube of mammals arises as two outgrowths from the 

 yolk-sac the fore-gut and hind-gut respectively. They are shown in 

 Fig. 241, A, which represents a young rabbit embryo placed in a vertical 

 position. Most of the spherical yolk-sac has been cut away. Anteriorly 

 the fore-gut (pti) is seen extending from the yolk-sac to the oral plate; 

 posteriorly the sac has given rise to a short hind-gut from which a tubular 

 ventral outgrowth, the allantois, has begun to develop. The allantois 

 will be described with the membranes which surround the embryo. In 

 an older stage (Fig. 241, B) the fore-gut and hind-gut have elongated, 

 and the connection of the tube, which they form, with the yolk-sac is 

 becoming reduced to a slender stalk. The entodermal tube within the 

 stalk is called the mtelline duct. Posteriorly the intestine and allantois 

 unite and form the cloaca, which is closed to the exterior by the cloacal 

 membrane.. (The marked bend in the intestinal tube shown in Fig. 241, B, 

 which is often seen in human embryos, is exaggerated, if not produced 

 altogether, by a post-mortem sagging of the yolk-sac.) 



