MIXED ORAL GLANDS. 193 



by this action of the elastic membranes. The nerves are sympathetic 

 fibers from the submaxillary ganglion and microscopic ganglia along the 

 ducts. The chorda tympani does not send fibers directly to the gland cells. 

 Sensory nerves may be derived from the branches of the mandibular nerve. 

 In the oral glands, not infrequently degenerating lobules occur, char- 

 acterized by abundant connective tissue between tubules with wide lumens 

 and low gland cells. Sometimes they are surrounded by leucocytes. 



THE DEVELOPMENT OF THE DIGESTIVE TUBE. 



The early development of the entoderm has been described in the 

 section on general histogenesis (page 18). At first it forms a layer lining 

 the blastodermic vesicle. Then by a process of folding and constriction 

 the 'pharynx' develops from its anterior part so that the entire entoderm 

 is shaped somewhat like a chemist's retort. The bulbous expansion is 

 the lining of the yolk sac. An analogous stage has been described in the 

 chick embryo (Fig. 20), where, in place of a thin walled yolk sac, there is 

 a solid mass of yolk-laden entoderm. From the posterior wall of the yolk 

 sac an entodermal outpocketing is produced, which rapidly becomes long 

 and slender. It is called the allantois (Fig. 220, al.}. At first the allan- 

 tois is directed posteriorly but soon it swings ventrally and then, as in C ? 

 it passes from the hind end of the digestive tract along the ventral body 

 wall into the umbilical cord. The part within the cord becomes a strand 

 of cells. Within the body, that portion of the allantois which is toward 

 the umbilicus or navel, becomes subsequently a fibrous remnant, the 

 urachus, which leads from the navel to the bladder. The bladder is the 

 dilated lower part of the allantois, and is therefore lined with entoderm, 

 being embryologically a part of the digestive tube. 



In mammalian embryos the allantois and the intestinal tract connect 

 freely at their posterior ends, and the entodermal area common to both is 

 called the cloaca. Here the entoderm comes in contact with the ectoderm 

 and forms the cloacal membrane, a structure comparable with the oral 

 membrane. After this membrane disappears there is no apparent line 

 of separation between the ectoderm of the skin and the entoderm of the 

 cloaca. In this region in both sexes a conical elevation, the genital papilla, 

 is formed, and the cloaca with its lateral walls closely approximated is 

 found within it. Gradually the allantois becomes divided from the intes- 

 tinal tract as shown in Fig. 220, B, C, and D. The mesenchymal tissue 

 between them thus comes in contact with the ectoderm to produce the 

 perineum which divides the cloaca into the urogenital sinus ventrally and 

 the anus dorsally. In E, the bladder is seen to terminate in the urethra 

 which in the male is considered to be chiefly an elongation of the ecto- 

 13 



