THE F(ETUS. 1021 



have been separated for the purpose ot studying them, but which are, in reaUty, 

 only two portions of the same membrane that forms a complete sac." ^ 



The cavity of this sac communicates with the bladder by means of a canal 

 divisible into two portions : 1. The infutuUbulum — a wide canal continued in 

 the amniotic portion of the umbiUcal cord, the walls of which are continuous, on 

 the one part, with the amniotic lamina of the membrane, and on the other part 

 with the chorial lamina, after being prolonged as a sheath around the cord. 

 2. The urachus, a musculo-membranous canal that extends from the umbilicus 

 to the summit of the bladder, with the umbihcal arteries on each side (Figs. 556 ; 

 557, A B). 



Structure and Development. — The allantois has the structure of a serous 

 membrane. It possesses : 1. An endothelial lining, the cells of which are thin 

 and polygonal, leaving between them, here and there, stomata that permit the 

 blood-serum to transude through its texture ; these cells contain the glycogen. 

 2. A middle layer or stroma, of the nature of connective tissue, little vascular, 

 and the fibres of which are parallel. 3. An external layer of loose connective 

 tissue, rich in vessels, and resembling subserous tissue ; this layer is continuous 

 with the parietal fibrous tissue of the foetal peritoneum, through the medium of 

 Wharton's jelly. 



The allantois, as we have described it, passes beneath the mucous membrane 

 of the bladder instead of being confounded with it. 



The blastodermic laminae — internal and middle — concur in the formation of 

 the allantois ; but the middle one has the largest share in this. There is a 

 tendency to beHeve that the extra-foetal portion of this vesicle is a cavity formed 

 in the intra-annexial connective tissue, and that this cavity is at a later period 

 transformed into a serous one. In any case, it serves as a support to the vessels 

 passing between the foetus and placenta. 



Allantoid Fluid. — This cavity contains a fluid, the quantity of which 

 increases with age. Colourless and transparent at first, it afterwards becomes 

 amber-tinted, and this gradually deepens ; at the same time it grows turbid, and 

 flocculent masses form in it. Its reaction is alkaline, and it has the property of 

 emulsifying fats. It contains albumen, urea, various salts, and sugar — 2*5 per 

 1000 average. The proportion of the latter does not vary much during 

 gestation. 



The presence of urea in this fluid, as well as the communication of the urachus 

 with the bladder, has led to the supposition that the urinary secretion of the 

 foetus contributes to its formation. But this is not demonstrated, and the mere 

 presence of urea is not sufficient to attribute this origin to the allantoid fluid ; 

 for the blood, lymph, cerebro-spinal fluid, as well as all other serous fluids, contain 

 a proportion at least as considerable. 



Hippomanes. — This name is given to small brown masses, more or less 

 numerous — though often there is only one — which float in the allantoid fluid. 

 "These bodies, of the consistency of gluten, and elastic like it, are flattened, 

 thinner at the borders than towards the centre, oval or irregularly circular, and 

 about the diameter of a five-franc piece. It is difficult to explain the presence of 

 the hippomanes in the allantoid sac. Nothing in its appearance indicates that it 

 may be formed at the expense of the liquid contained in this membrane. Some- 

 times pediculated hippomanes are found, and these may assist in explaining the 

 formation of the free hippomanes. Bourgelat speaks, in his Anatomic, of pedicu- 



• F. Lecoq, Bes Annexes du Foetus. 

 67 



