THE PRIMITIVE ALIMENTARY CANAL. 



impar and not from between the tuberculum impar and the ventral ends of the 

 hyoid arches. 



Derivative of the Dorsal Wall (Seessel's Pouch). The dorsal diverticulum 

 from the cranial end of the fore-gut, to which the above term is applied, enters the 

 base of the occipital region of the primitive head. The ultimate fate of the pouch 

 is unknown in the human subject, but it has been suggested that it is represented 

 by a depression in the mucous membrane of the cranial part of the pharynx, close 

 to the pharyngeal tonsil, which is known as the pharyngeal bursa. 



The reader who has followed this description will have noted that from the 

 cranial portion of the fore-gut are formed the caudal or inferior part of the mouth 

 (with the exception of the lips, teeth, and gums), the pharynx, the thyreoid gland, 

 the thymus, the parathyreoids, the respiratory organs, and the oesophagus. The 

 more caudally situated portion of the fore-gut is differentiated into the stomach 

 and the first and second parts of the duodenum. 



The stomach is formed from the part of the fore-gut immediately adjacent to 



Rudiment of thyreoid gland Trachea 



Notochord 

 Medulla spinalis 

 Ectoderm of embryo 



Foramen ctecum 



i Oesophagus 



Stomach 



Pancreas rudiment 



Peritoneum 



Cerebellar part 

 of hind-brai 



Hypophysis . 

 Mid-brain 



Mesoderm of . 

 amnion 



Ectoderm of 

 amnion 



Cerebral hemisphere 



toneum 



Descending 

 colon 



uctus deferens 

 rinary bladder 

 reter 



todoeum 



<3enito-urinary 

 chamber 



Allan toic 

 diverticulum 



Chorion 



Mandibular arch 



Pericardium 

 Liver diverticulum 



Mesoderm of placenta 



! Yolk-sac 

 Diverticulum of peritoneum 



FIG. 63. SCHEMA showing complete separation of cloaca into dorsal and ventral parts and the temporary 

 ventral hernia of a portion of the gut through the umbilical orifice. The heart is not shown. (After 

 Mall, modified.) 



the oesophagus, and the duodenum from the more caudally placed portion, which 

 is directly continuous with the mid-gut. 



The Liver and Pancreas. When the embryo is about three weeks old and has 

 attained a length of 2*5 mm. a ventral diverticulum appears in the ventral wall of 

 the duodenal part of the fore-gut, and when the age of the embryo is about four 

 weeks and its length increased to about 4 mm. a diverticulum is formed in the 

 dorsal wall a little nearer the cranial end. The ventral pouch is the rudiment of 

 the liver, the gall bladder, the bile- ducts, and a portion of the pancreas, and the 

 remainder of the pancreas is formed from the dorsal diverticulum (Figs. 57, 62, 63). 



The Derivatives of the Mid-Gut. The mid-gut is that part of the primitive 

 alimentary tract which lies between the more definitely enclosed fore-gut and 

 hind-gut, and it is in free communication with the yolk-sac by the vitello-intestinal 

 duct. It is transformed into the greater part of the small intestine. 



The Derivatives of the Hind-Gut. The parts formed from the hind-gut 

 are : (1) The terminal part of the ileum ; (2) the whole of the large intestine, 

 except a small portion of the anal canal ; (3) the urachus, the urinary bladder, the 

 urethra in the female, and the greater part of the urethra in the male. 1 



1 T. B. Johnston, Journ. ofAnat., Oct. 1913 ; H. v. Berenberg-Gossler, Anat., Heft. 1913. 



