ALLANTOIS 



55 



thus formed is drawn out into a long tubular passage, the vitelline duct, which 

 widens distally into a rounded vesicle called the umbilical vesicle. 



Allantois. In all the Primates the vesicular allantois of lower forms is 

 represented merely by a narrow tubular passage imbedded in the mesoderm of 

 the connecting stalk. It appears as a recess of the posterior wall of the yolk-sac 

 at a very early stage, before the formation of the hind-gut (figs. 53, GO, 61, 79). 

 This recess is drawn out into a tube as the connecting stalk increases in length. 



ect 



FIG. 80. DIAGRAMMATIC LONGITUDINAL SECTIONS THROUGH THE EMBRYO OF THE RABBIT. THE 

 SECTIONS SHOW THE MANNER IN WHICH THE PRO-AMNION IS FORMED BY A DIPPING DOWN OF 



THE HEAD AND ANTERIOR PART OF THE BODY INTO A DEPRESSION OF THE BLASTODERM, WHICH 

 AT THIS PART IS FORMED OF ECTODERM AND ENTODERM ONLY. THE DIAGRAMS ALSO ILLUSTRATE 

 THE MODE OF FORMATION OF THE ALLANTOIS AND OF THE TAIL-FOLD OF THE AMNION IN THIS 



ANIMAL. (Van Beneden and Julin.) 



ect, ectoderm ; ent, entoderm ; me, mesoderm ; cce, parts of the coelom; cce', pericardial ccelom, the 

 heart not being represented; pr.a., pro-amnion; pi, seat of formation of the placenta; all, allantois ; 

 am, amnion. 



When the stalk is displaced to the ventral aspect, and the umbilical cord is 

 formed, the passage persists for a time in the cord, while its in tra- embryonic 

 portion becomes the urachus. 



In lower mammals the entodermic diverticulum varies much in the degree of its development. 

 In the ungulates and carnivores it forms a large vesicle ; in most rodents (fig. 80) it is less 

 extensive, being confined to the placental site ; in the guinea-pig it is reduced to a tubular 

 passage in the body-wall and the stalk is a solid cord of mesoderm ; but in all below Primates 

 the diverticulum, with its covering layer of mesoderm, projects free into the extra- 

 embryonic ccelom before it comes into contact with the chorion. In the Primates the 

 embryonic shield is connected from the first with the chorion by the mesodermic 

 connecting stalk, and the allantois never projects free into the ccelom. The chorion is thus 

 vascularised directly and not through the agency of the allantois. This close attachment of the 



