.1630 A MANUAL OF ANATOMY 



caudal or posterior end of the embryo, their opposed surfaces, which 

 consist of mesoderm, being here connected. From this mesoderm 

 a peduncle is developed, and this mesodermic peduncle constitutes 

 the allantoic stalk, in the centre of which is the remnant of the 

 allantoic diverticulum, now without a lumen. After having tra- 

 versed the chorionic cavity it blends, as stated, with the chorion 

 frondosum, which gives rise to the foetal part of the placenta. The 

 mesodermic tissue of the allantoic stalk contains the allantoic or 

 umbilical bloodvessels (two arteries and two veins). This vascular 

 stalk therefore establishes an important communication between 

 the caudal or posterior extremity of the embryo and the foetal 

 (chorionic) part of the placenta. Its vessels render the placental 

 area of the chorion, and also the chorionic villi, vascular. 



Whilst the amnion and chorion are extra-embryonic, the allantois 

 is both intra-embryonic and extra-embryonic. 



Functions of the Allantois. — ^The extra-embryonic part or allantoic 

 stalk is concerned with respiration and nutrition, as regards the 

 human embryo, through means of the allantoic arteries and veins. 

 In the embryos of oviparous animals it also serves the purpose of 

 excretion (urinary). 



Subsequent History of the Allantois. — The extra-embryonic part 

 contributes fo the formation of the chorionic or foetal part of the 

 placenta. The intra-embryonic part, or allantoic diverticulum, 

 which is originally tubular, is connected, as stated, with the ventral 

 or urogenital compartment of the cloaca. The urinary bladder is 

 to be regarded as developed from the anterior or cephalic portion 

 of this compartment, and perhaps from the root or cloacal end of 

 the allantoic diverticulum. The portion of the allantoic divertic- 

 ulum between the apex of the urinary bladder and the umbilicus 

 loses its tubular character, and persists in the form of a fibrous 

 cord, called the uracbus. 



The Umbilicus and Umbilical Cord. 



The umbilicus is the opening in the ventral body-wall, through 

 which originally the vitelline duct or yolk-stalk enters the ccelom 

 or body-cavity, of the embryo. The allantois also leaves the foetal 

 body through it. As the somatopleure (somatic mesoderm and 

 ectoderm) closes the ventral body-wall it divides the embryonic 

 portion of the body-cavity from the extra-embryonic portion. 

 When the somatopleure reaches the situation of the vitelline duct 

 an aperture is left in it for the duct, this aperture being the primitive 

 umbilicus. After entering the embryonic body-cavity, the vitelline 

 duct is attached to the ventral aspect of the mid-gut. At the 

 margins of the umbilicus the somatopleure becomes continuous 

 dorsalwards with that of the amnion, and ventralwards it ensheathes 

 the vitelline duct for a short distance towards the yolk-sac, which 

 is extra-embryonic. 



The un^bil^p^al cord connects the foetus with the placenta, and, 



