THE EMBRYO. 



1203 



in front of the posterior end of the hind-gut. This invagination is termed the 

 proctodce.um ; the mesoblast between it and the hypoblastic lining of the hind- 

 gut is thinned, and ultimately the septum breaks down and disappears, and the 

 hirid-gut opens on the surface ; into this part of the 

 hind-gut the urinary and generative organs open for 

 a time, and so it constitutes a common cloaca. The 

 small portion of the hind-gut behind the orifice of the 

 anus is named the caudal or post-anal gut ; it com- 

 municates with the neural tube by means of a canal, 

 the neurenteric canal, already referred to. Ultimately 

 the post-anal gut becomes obliterated, and it, together 

 with the neurenteric canal, finally disappears. 



The peritoneal cavity is the space .left between the 

 visceral and parietal layers of the mesoblast, and the 

 serous membrane is developed from these layers. 



The tongue originates from the floor of the pharynx. 

 The anterior or papillary portion first appears as a 

 rounded elevation, the tuberculum impar, between the 

 ventral ends of the mandibular and hyoid arches. Be- 

 tween the third and fourth arches a second larger ele- 

 vation arises, in the centre of which is a median groove 

 or furrow. This second elevation is termed the fur- 

 cula, and from it the epiglottis is developed, while the 

 median furrow becomes the entrance to the larynx (Fig. 

 771). The tuberculum impar and the furcula are at 

 first in apposition, but are soon separated by a ridge 

 produced by the forward growth of the second and third 

 arches. This ridge gives rise to the posterior part of 

 the tongue and extends forward in the form of a V, so as 

 to embrace between its two limbs the tuberculum impar. 

 At the apex of the V there is a pit-like invagination to 



form the middle thyroid rudiment, and this depression persists as the foramen 

 caecum of the adult. The union of the two parts of the tongue is indicated even 



Septum transversum. 



FIG. 766. Final disposition 

 of the intestines and their vas- 

 cular relations. (Jonnesco.) A. 

 Aorta. //. Hepatic artery. S. 

 Splenic artery. M. Col. Branches 

 of superior mesenteric artery. 

 m, m'. Branches of inferior 

 mesenteric artery. 



Liver.- 



Mesogasti'ium } 

 (anterior part) 



Umbilical vein. 



Umbilical cord 



Aorta. 



Mesogastrium 

 (posterior part). 



Stomach. 



Intestinal 



V -shaped loop. 

 Mesentery. 



Colon. 



^e^ 



FIG. 767.-The primitive mesentery of a six weeks' human embryo, half schematic. (Kollmann.) 



in the adult by a V-shaped depression, the apex of which is at the foramen caecum, 

 while the two limbs run outward and forward parallel to but a little behind 

 circumvallate papillae, which are therefore developed from the tuberculum impar 

 (Figs. 772, 773). The tonsils are developed from the second branchial 

 make their appearance between the fourth and fifth months. 



