896 



EMBRYOLOGY. 



placental tufts. It adheres to the internal surface of the uterus. Between 

 the two membranes there is found a small quantity of sanguinolent fluid. 



The internal face, lined by the external layer of the allantois, is united 

 in the closest manner to that membrane, except at the umbilical cord, where 

 there exists a kind of conical infundibuluin occupied by the umbilical 

 vesicle. 



STRUCTURE. The chorion is a delicate fibrous membrane, traversed by 



the vascular ramifications of the placenta. It is formed by the vitelline 



membrane, to the inner face of which the serous vesicle is applied. It is 



stated that the zona pellucida, or primitive chorion, as it is called, disappears 



when the serous vesicle, the definitive chorion, becomes 



Fig. 423. independent of the amnion. 



^ The chorion is destitute of vessels mAil the 



allantois is developed. 



2. The Amnion. (Figs. 423, 424.) 



The second sac enveloping the foetus, the amnion 

 floats freely in the interior of the chorion, to which 

 it is only united at one point through the medium of 

 the umbilical cord. It contains the young creature, 

 which is also attached to its inner face by the vessels 

 NION AND ALLANTOIS wi ""^ ~ VJL "' ^ * s ova ^ i n shape, and has thin trans- 

 a, Umbilical vesicle ; 6, parent walls. 



Amniotic cavity ; c, Al- Its external face is covered by the inner layer of 



lantois. the allantois, to which it adheres slightly. The 



internal face is perfectly smooth, and is applied more 



or less directly to the skin of the foetus. It exhales a fluid in which the 

 latter floats the liquor amnii. 



Liquor amnii. Inclosed with the foetus in the 

 cavity of the amnion, this fluid is more or less abun- 

 dant, according to the period of gestation ; its relative 

 quantity being always less as the foetus is advanced 

 in development. At an early period it is somewhat 

 milky in appearance, but later it assumes a citrine or 

 slightly reddish tint. It has a salt taste, and con- 

 tains 99 per cent of water, with albumen and salts, 

 the principal of which are chloride of sodium and the 

 sulphate and phosphate of lime. 



FECUNDATED EGG, SHOW- w ] 



ING FORMATION OF AM- Qf ^ ^^ 



FECUNDATED EGG OF FOWL 

 WITH ALLANTOIS NEARLY 

 COMPLETED. 



a, Inner lamina of am- 

 niotic fold ; 6, Outer 

 lamina of the same; c, 

 Point where the amnio- 

 tic folds come in contact 

 with each other ; the 

 allantois is seen pene- 

 trating between the 

 inner and outer laminae 

 of the amniotic fold. 



3. The Allantois. (Fig. 423.) 



The allantois is a membrane that covers the inner 

 face of the chorion, and is folded around the insertion 

 of the umbilical cord, to spread itself over the whole 

 external surface of the amnion. It thus transforms 

 the chorial sac into a kind of serous cavity, in which 

 the amnios is inclosed as a viscus. 



The inner, or amniotic lamina, is attached to the 

 amnios "so slightly that dissection, and especially 

 insufflation, easily destroys its adhesion. When the 

 second of these measures is resorted to, in order to 

 separate the two membranes, the allantoid surface assumes a bosselated 



