704 THE FCETAL MEMEEANES. 



a peripheral extension of the pleuro-peritoncal cavity. The embryo thus 

 comes to sink down as it were (the cephalic part before the rest) into 

 the hollow produced by the rising of the amniotic folds round it. 



The backward folds deepening more and more, gradually conrerge 

 on the dorsum of the embryo, and at last come together (fig. 512, 3), the 

 margins of the reflection narrowing rapidly and being finally completely 

 obliterated or lost by their convergence and by the subsequent dissociation 

 of the inner from the outer divisions of the folds (fig. 512, 4). The sepa- 

 rated inner division now becomes the entire closed sac of the amnion, 

 connected only with the rest of the parts at the umbilical constriction 

 where it is continuous with the integument of the embryo. The outer 

 dissociated division is the false amnion of Pander and Von Baer, passing 

 out into the remaining peripheral part of the blastoderm, and con- 

 stituting for a time an external covering of tlie ovum, -which in birds 

 and reptiles appears afterwards to be lost by thinning or absorption ; 

 but which in mammals may be connected with the development of the 

 permanent chorion in a manner to be referred to hereafter. 



Fig. .^14. — Human Embryo of betwken 

 THE Third and Fourth Week, Mag- 

 nified ABdUT FIVE DiAMETERS (frOUl 



Koiliker after Allen Tliomson). 



a, amnion adherent (unusually) to the 

 interior of the chorion in the dorsal 

 region ; h, umbilical vesicle or yolk-sac 

 with a wide communication with the 

 intestine ; c, lower jaw ; d, superior 

 maxillary process ; c, second j)Ostoral 

 plate, and behind it other two, with 

 the pharyngeal clefts behind each ; 



f, commencement of the anterior limb ; 



g, primitive auditory vesicle ; h, eye ; 

 i, heart. 



In the human ovum, as in most mammals, the amnion is formed at a 

 very early period. The membrane lies at first so close to the embryo 

 that it is with difficulty distinguished from the surftice of the body : 

 but after the dorsal closure is completed, it is soon separated by the 

 fluid which accumulates in its cavity. 



The muscular contractility possessed by the amnion doubtless resides in its 

 outer layer derived from the somatopleui-e. The contractions appear to be 

 rhythmic, as they may be seen in the opened incubated egg of the fowl, or even in 

 the entire egg. by means of a bright light in a dark chamber, from the sixth or 

 seventh day of incubation ; and it is probable that they are of a similar natui'e 

 in mammals. 



The amniotic fluid contains about 1 per cent, of solid matter, consisting chiefly 

 of albumen, but also traces of urea, which is probably derived from the urinary 

 secretion of the foetus. 



It would appear that there is a, difference in the structure of the reflected or 

 false amnion in birds and in mammals. In the former it is composed of the 

 same two layers as the amnion itself, but in mammals the development of the 

 jnesoblast appears to cease at the place of reflection of the true into the false 

 amnion, so that the latter consists only of the coi-neous layer or epiblast. 



The Allantois : Urinary Vesicle. — Although this membrane 

 "becomes in the more advanced stage of development widely distributed 



