DEVELOPMENT OF THE DIGESTIVE SYSTEM. 1243 



It has been further shown that the three portions are at first separated from one 

 another by septa, the pharyngeal membrane and the proctodeal membrane, respectively, 

 but the septa disappear at an early date, and the three parts are thrown into continuity. 



There is thus formed a tube, the primitive alimentary canal, extending through the 

 body from the mouth aperture in front to the anal orifice posteriorly. 



The entodermal segment of the primitive alimentary canal is divided into three 

 portions as follows : 



(1) That portion which is enclosed within the head fold is termed the foregut ; the 

 portion within the tail fold is termed the hindgut ; and the intermediate portion is termed 

 the midgut. The midgut at first lies opposite the communication with the yolk sac, and 

 the other portions cephalic to and caudal to this level. 



From the foregut are formed the posterior part of the mouth, the pharynx, oesophagus, 

 stomach, and the greater part of the duodenum. From the mid and hindgut are formed 

 the rest of the small intestine, and the whole of the large intestine, as far as to the 

 " white line " of the anal orifice. There is no sharp limit between the mid and hindgut 

 or between the portions of the intestinal canal formed from them (see p. 47). 



The different parts of the tube become modified in their structure, so as to form 

 special organs, and in many regions outgrowths occur in the form of hollow diverticula, 

 from which the accessory glands are formed, which lie outside the wall of the original 

 tube. Organs such as the liver, pancreas, and salivary glands arise in this way. 



It should be stated here, also, that this primitive intestinal tube forms the basis, 

 not only of the alimentary canal and its associated organs, but it also is the source from 

 which many other organs, not ultimately connected with digestion, are formed. 



Thus, the respiratory tract below the level of the orifice of the larynx is formed as 

 an outgrowth from the ventral wall of the primitive foregut, and remains permanently 

 connected with it at that point, though in structure and function it becomes very 

 different from the tube from which it is derived. 



Other structures also, namely, the thyreoid gland, the parathyreoid glands, and the 

 thymus, are formed from the alimentary canal as diverticula, but they eventually loose 

 their connexion with the wall of the tube, and become specialised in function and in 

 structure. 



Furthermore, the allantois, a diverticulum from the hindgut, is an important rudi- 

 mentary organ, and a part of the primitive hindgut is cut off from the primitive cloaca 

 to form the urinary bladder and a portion of the urethra. 



Accounts of the development of the organs mentioned which are not connected with 

 the alimentary canal in the adult will be found in the sections dealing with them. 



Development of Mouth and Pharynx. The development of the mouth and of 

 the pharynx from the oral sinus and anterior part of the foregut are intimately 

 associated with one another, and with the formation of the mouth and nose. 



The stomodaeum or oral sinus first appears as a depression situated between the 

 primitive forebrain above and the pericardial region below. 



The floor of this depression is formed by the pharyngeal membrane, which consists of 

 ectoderm and entoderm only, mesoderm being absent; the membrane separates the oral sinus 

 from the anterior end of the foregut, but at an early stage it ruptures and disappears. 



The mouth cavity of the adult is formed in part from the oral sinus, and in part 

 from the anterior end of the foregut, or primitive pharynx. The line of division between 

 the portions of the mouth derived from these two parts is difficult to trace, on account 

 of the very extensive changes which occur after the pharyngeal membrane has disappeared, 

 and which are associated with the formation of the face and of the nose. The portion of 

 the mouth cavity derived from the primitive pharynx is lined with entoderm, and that 

 from the oral sinus with ectoderm. The position of the original pharyngeal membrane 

 may be represented by an imaginary plane extending from the anterior part of the body 

 of the sphenoid to the base of the alveolar process of the mandible on its lingual surface. 



Thus, the cavity of the nose is derived from the upper part of the oral sinus, while the 

 floor of the mouth is formed from the pharynx. The adamant (O.T. enamel) of the 

 teeth and the secreting epithelium of the parotid gland are ectodermal structures, while 

 the epithelium of the tongue and submaxillary and sublingual glands is entodermal 

 in origin. 



The ectodermal or oral sinus portion of the mouth, then, gives origin to the lips, 

 teeth, and parotid glands; while in the pharyngeal portion are developed the tongue, 

 submaxillary and sublingual glands. 



The upper lip is formed from the tissues covering the frontal and maxillary processes 

 (see development of face). 



