72 D. NOEL PATON, B. P. WATSON, AND J. KERR ON 



surface of the foetus it communicates directly with the amniotic vesicle. When later 

 the opening of the bowel differentiates from the urinary orifice, both anus and the 

 meatus urinarius are in direct communication with the vesicle. 



In reptiles, birds, and mammals another vesicle filled with fluid develops to a greater 

 or less extent. This is the allantois. It is an outgrowth from the posterior end of the 

 gut, which grows out in the mesoblast surrounding the amnion, and it may finally 

 separate the amnion from the periphery of the ovum. From the first it is essentially 

 a continuation of that part of the hind gut which will afterwards form the urinary 

 bladder of the foetus. In the wall of this vesicle, in most mammals, the blood-vessels 

 of the embryo form a connection with the lining membrane of the uterus and produce 

 a placenta. 



In most of the lower mammals both vesicles are well developed ; but in man and the 

 anthropoid apes the allantois attains a very insignificant size and in man takes no part 

 in the formation of the placenta. 



The source of the fluids which fill these vesicles is still doubtful. In reptiles and 

 birds they are necessarily fcetal in origin. The occurrence of urea and uric acid in the 

 allantoic fluid of these animals, as was long ago demonstrated by Jacobson and Prevost 

 (Gusserow, Arch. f. Gynaekologie, Bd. iii. p. 244, 1872), shows that in them the 

 allantoic fluid is derived, in part at least, from the kidneys. 



In mammals the question is not so simple, since the membranes surrounding one or 

 both of the vesicles are in close relationship to the mucous membrane of the maternal 

 uterus. The possibility is thus suggested that one or both fluids may be derived from 

 the mother — a view which has been upheld by one set of authorities, and which is very 

 commonly accepted in modern text-books of obstetrics. Other observers maintain that 

 the fluids are fcetal in origin in the mammal as in the sauropsida. 



A study of the anatomical relations of the membranes makes it difficult to accept 

 the former hypothesis without very conclusive evidence in its favour. In most mammals 

 the membrane of the allantois alone is in relation to the maternal mucous membrane ; 

 but it is always definitely separate from it, and is easily detachable. 



The only connection between the fcetal and maternal circulation is through the 

 placenta, which, although it varies greatly in distribution and complexity, is always 

 essentially the same in structure — consisting of tufts of fcetal blood-vessels, carried out 

 in the allantoic mesoblast, which have thrust their way into blood-spaces in the mucous 

 membrane of the maternal uterus, so that the fcetal blood is brought into intimate 

 relation with the maternal blood. 



In all mammals fcetal blood-vessels run throughout the wall of the allantois — 

 both in that part lying against the mucous membrane of the uterus, but separated 

 from it by the chorionic amnion, and also in that part which surrounds the 

 amniotic sac. 



In man, while in the very early stage of development the ovum is attached all 



