THE DEVELOPMENT OF MAMMALS 543 



embryo. The archenteron remains connected with the yolk sac by a 

 narrow stalk that extends through the umbilical cord. The anterior 

 part of the archenteron, the foregut, differentiates into the pharynx, 

 esophagus, stomach and a small portion of the duodenum. The rest of 

 the archenteron, the hindgut, forms most of the intestinal region and 

 much of the embryonic cloaca. Only the linings of these organs are 

 endodermal; the connective tissue and muscles in their wall are derived 

 from mesoderm. 



The pharyngeal pouches, thyroid gland, trachea and lungs develop 

 as outgrowths from the pharynx, as described in section 218. A ventral 

 outgrowth from the posterior end of the foregut differentiates into the 

 liver and much of the pancreas, but part of the pancreas develops as a 

 separate dorsal outgrowth (Figs. 31.5 and 31.6). This explains why the 

 pancreas has two ducts, one entering the intestine with the bile duct 

 and one independently. 



The most anterior and posterior ends of the digestive tract develop 

 from ectodermal pockets that invaginate and meet the archenteron. 

 Initially, plates of tissue separate these pockets from the archenteron, 

 but these plates eventually break down. The lining of the mouth, the 

 enamel of the teeth and the secretory cells of the salivary glands are 

 ectodermal in origin. The anterior and intermediate lobes of the pitui- 

 tary gland develop as an ectodermal evagination from the roof of the 

 mouth pocket, as described in Chapter 30, but the posterior lobe of 

 the pituitary develops as an evagination from the floor of the dien- 

 cephalic region of the brain. Part of the embryonic cloaca is of ecto- 

 dermal origin. A cloaca persists in the adults of most vertebrates, but is 

 divided in most mammals to form the rectum and parts of the urogenital 

 passages (part of the urethra in the male; part of the urethra and vagina 

 in the female). 



263. DiflFerentiation of the Mesoderm 



As the mesoderm spreads out from the primitive streak, its lateral 

 portion splits into two lavers (Fig. 31.6). This part of the mesoderm is 

 known as the lateral pSate, and the space between the two layers is the 

 embryonic coelom. The embryonic coelom is continuous with the large 

 extraembryonic coelom, or chorionic cavity, until the folding processes 

 described above separate the embryo from surrounding structures. The 

 inner layer of the lateral plate mesoderm, which lies next to the archen- 

 teron, forms the connective tissue and musculature (visceral muscles) of 

 the digestive tract, the visceral peritoneum and the mesenteries. The 

 outer layer forms the lateral wall of the coelom, that is, the parietal 

 peritoneum, and may contribute to the musculature of the body wall. 



Unlike the lateral plate, the mesoderm on each side of the neural 

 tube and notochord becomes segmented and forms a series of paired 

 somites Some of the mesoderm of the somites spreads out beneath the 

 surface ectoderm to form the dermis of the skin, some migrates around 

 the neural tube and notochord and differentiates into the vertebral 

 column and much of the skull, and the rest forms the segmented, em- 



