DIVISION OF THE GERMINAL MEMBRANE. 



527 



relation with the maternal blood-sinuses, by the tufts dipping Functions of 

 down or being enveloped therein, that the completest con- the P lacen t a - 

 tact and facility of exchange, but not of intermixture, may be insured. 

 Things are arranged in such a way that the maternal and foetal blood do 

 not intermingle, each being confined in vessels of its own, through the thin 

 walls of which nutritious matter may pass and excrementitious matter re- 

 pass. Every foetal tuft has a deciduous layer upon it, and the blood 

 brought by the curling arteries of'the uterus furnishes to the foetus its ox- 

 ygen, and receives back carbonic acid, with other excrementitious matters. 

 In this respect, respiration is carried on by the aid of a mechanism which 

 answers to the gills of fishes, the maternal arterial blood standing for the 

 aerated water ; but, besides this, the tufts have another duty to dis- 

 charge the obtaining of albumenoid material from the maternal blood. 

 The placental mechanism is therefore much more perfect in its action 

 than the. tuft mechanism which preceded it. 



The germinal membrane, formed as has been described, already ex- 

 hibits at one spot an opaque area of a roundish shape, con- change* in the 



Fiq 258 Sisting Of Cells and granules. To germinal mem- 



^ G brane,andpro- 



this the designation of germinal duction of lay- 

 area is given. At this area the ers - 

 membrane next becomes divisible into t\vo 

 laminaa, and eventually throughout its whole 

 extent, as seen in Fig. 258. Of these lam- 

 inae, the exterior, which is nearer to the zona 

 pellucida, is the serous layer. It is the raised 

 membrane of the figure, and in it are to be 

 developed the nervous and muscular systems 

 of the embryo. The interior is designated the mucous layer, and from 

 Fig. 259. this arise the digestive organs. 



The germinal area by degrees 

 loses its circular form and becomes 

 oval, its central portions clearing 

 off and giving rise to the area pel- 

 lucida. Around this the opacity 

 is increased, and in it blood-ves- 

 sels appear; hence to this dark cir- 

 cle the designation of area vascu- 

 losa is applied. In the pellucid 

 zone is next seen a delicate line, 

 the primitive groove, T ' he primitive 



Fig. 259. It OCCUrS groove and 



in the serous layer dorsal laminie ' 



The primitive groove, magnified 8 diameters. Only, IS Wider at one end than at 



