250 EMBRYOLOGY. 



surface appears thin and translucent, consists of (1) a connective-tissue 

 membrane, and (2) an epithelial covering, which is identical with the 

 original serosa. 



The connective-tissue membrane possesses at first the character of 

 embryonic mucous tissue, and exhibits therefore branched stellate cells 

 in a homogeneous matrix. Subsequently the mucous tissue is con- 

 verted, as at other places in the body, into fibrous connective tissue. 



The epithelium of the chorion consists in the first months, according 

 to the statements of KASTSCHENKO and SEDGWICK MINOT, of two 

 layers a superficial one, in which no cell-boundaries are visible 

 (protoplasmic layer), and a deeper one, in which the individual cells 

 are distinctly separated. Additional particulars are given in the 

 description of the placenta. 



The embryonic adjuncts enclosed within the chorion the amnion 

 and yolk-sac undergo in Man during pregnancy the following 

 changes. 



2. The Amnion. 



The amnion (am] immediately after its origin lies close on the 

 surface of the embryo (fig. 144), but soon becomes distended by the 

 accumulation of fluid, the liquor amnii, in its cavity (fig. 132 5 ). It 

 increases to a much greater extent than in other Mammals, in which 

 it is often found to be smaller than the allantoic sac (compare the 

 foetal membranes of the Rabbit, fig. 133). Finally, in Man it Jills 

 out the entire blastodermic vesicle, since it everyiohere applies itself 

 (fig. 143 am) closely to the inner -wall of the chorion (ch). 



Its wall is rather thin and translucent, and also consists, like the 

 chorion, of an epithelial and a connective-tissue layer. 



The epithelium, derived from the outer germ-layer of the embry- 

 onic fundament, lines the amniotic cavity within, and is continuous 

 with the epidermis of the embryo at the dermal navel ; at the place 

 of transition it is composed of layers ; but else\vhere it is a single sheet 

 of pavement cells. The connective-tissue layer is thin and at the 

 navel continuous with the corium. 



The amniotic orfcztal water is slightly alkaline, and contains about 

 I/ solid constituents, among which are found albumen, urea, and 

 grape-sugar. Its volume is greatest in the sixth month of pregnancy, 

 and it often attains a weight of not less than a kilo [2'2 Ibs. avoir- 

 dupois] ; then it diminishes to about one-half that amount at the 

 time of birth, and in the same ratio as the embryo by its increased 

 growth demands for itself more room. Under abnormal circumstances 



