Jf'I'/-:\f>.lf,'ES OF THE FCETCS. 83 



become capillaries. If, instead of forcinfr the air towards the ramifica- 

 tions, it is sent in the contrary direction, it will be seen to pass to that 

 ])ortion of the membrane covering the umbilical cord, and insinuate 

 itself between the amnion and the layer of allantois enveloping it, thus 

 proving the continuity of the membrane. 



As has been stated, the cavity of this sac is in communication with 

 the interior of the bladder by means of the nracJnis — a narrow canal in 

 the amniotic portion of the umbilical cord, and which widens at the 

 origin of the allantoidean portion, where its walls are continuous with 

 the amniotic layer of the membrane, as well as the chorial layer, after 

 being prolonged as a sheath around the cord. These arrangements 

 show the allantoid cavity to be a kind of urinary reservoir or dependency 

 of the bladder, the fundus of which is prolonged in an infundibular 

 manner as far as the umbilicus, to constitute the urachus, which follows 

 the umbilical vessels in the amniotic portion of the cord, and ends by 

 forming the allantoid cavity. 



The structure of this membrane is slightly fibrous, with a layer of 

 epithelium ; it is thin and pellucid, and appears to be destitute of 

 vessels at a late period of fcetal life. It bears on its surface the umbilical 

 vessels, from the umbilicus to the chorion. 



The allantois contains a fluid — the allantoic liquid — the quantity of 

 which, like that of the amnion, is greatest at an early period. It is 

 then colourless or slightly turbid ; but with the growth of the foetus it 

 gradually assumes a yellowish hue, until near parturition, when it is 

 brown. It presents somewhat the same physical properties as the 

 liquor amnii, and contains albumin, osmazone, a nitrogenous mucilagi- 

 nous matter insoluble in alcohol, a particular principle named allanto'inc, 

 which appears to be the urate of urea, with lactic acid, lactate of soda, 

 and phosphates of soda, lime, and magnesia. Allantoic acid is not 

 found in this fluid in Rolipeds. In addition to the large percentage of 

 water, there is a notable proportion of sugar at an early epoch of fcetal 

 life ; this, however, gradually and linally disappears towards the ter- 

 mination of gestation. 



It is probable that before the fa-tal circulation is fully established, the 

 allantoid fluid serves to nourish the young creature, but that towards 

 the end of gestation it is a product of the urinary secretion of the foetus. 

 It is certain that as gestation approaches its termination, the renal 

 excretion of the young animal passes from the bladder along the 

 urachus, and deposits near the allantoic orifice of that tube a thick fluid 

 of a reddisli colour, and possessing a urinous odour ; it contains uro- 

 erithrin and hippuric acid. 



The fluid contains, besides, whitish filaments, and small oval or 

 discoid masses of a brownish colour from the size of a pea to that of a 

 hen's egg, either floating about in the cavity or attached to the allantois 

 by a narrow pedicle. Sometimes they are very niimerous, and at other 

 times there is only one. From the fanciful notions attached to them in 

 ancient times, they were named the " hippomancs." Usually they have 

 the consistency and elasticity of gluten, are flattened, and are thinner 

 at the border than the centre. Those attached to tBe allantois are 

 generally pyriform, and their pedicle is narrower as they ai-e more 

 developed : proving that the loose bodies in the fluid were originally 

 appendages of the allantois. It is not improbable that they are inspis- 

 sated parts of the allantoic fluid which were originally deposited upon 

 the membrane. They contain much oxalate of lime. 



