GERMINAL MEMBRANE. 455 



by a more opaque circle (whose opacity is due to the greater accumula- 

 tion of cells and nuclei in that part), which subsequently becomes the 

 area vasculosa. In the formation of these two spaces, both the serous 

 and mucous layers of the germinal membrane seem to take their share ; 

 but the foundation of the vertebral column and nervous centres appears 

 to be laid chiefly if not entirely in the serous layer ; whilst the mucous 

 is afterwards concerned more especially in the formation of the nutritive 

 apparatus. Between these a third layer subsequently makes its ap- 

 pearance ; which, as the first vessels of the embryonic structure are 

 formed in it, is termed the vascular layer. 



808. Thus the first development of the Mammalian embryo is into 

 a sac, enclosing the store of nutriment that has been prepared for it, 

 in fact, a stomach ; and we shall presently see, that it is by the agency 

 of the walls of this sac, that the nutrient materials which it encloses 

 are prepared for being appropriated to the development of the more 

 permanent part of the fabric, which is to be evolved from the centre of 

 the mulberry mass. But we may here stop to notice the interesting 

 fact, that the development of the ovum in the lowest classes of animals 

 may almost be said to cease at this point ; the external layer of the 

 germinal membrane remaining as the integument ; the internal layer 

 becoming the lining of the stomach ; and the space occupied by the 

 yolk forming the digestive cavity, into which an entrance or mouth is 

 formed, by the thinning away of the germinal membrane at a certain 

 point, round which tentacula or prolonged lips are usually developed. 

 This is the essential part of the history of development in the simpler 

 Polypes ; and we see how remarkably it corresponds with the history of 

 development of the lower Cryptogamic plants, in which the first-formed 

 membranous expansion, or primary frond, remains as the permanent 

 leaf. In the Mammalia, on the other hand, the greater part of the 

 germinal membrane, and of the cavity which it forms, have a merely 

 temporary purpose ; being cast off, when they have performed their 

 function, like the cotyledons of Flowering Plants. 



809. During the time which is occupied by these important changes, 

 the Ovum passes through the Fallopian tubes, and makes its way into 

 the Uterus. During its transit through the Fallopian tubes, the Mam- 

 malian ovum, like the ovum of Birds in its passage through the ovi- 

 duct, receives an additional layer of albuminous matter secreted from 

 the walls of the tube ; and this is surrounded by a fibrous membrane, 

 whose structure and mode of formation have been described on a former 

 occasion ( 181, 182). The outer layer of this envelope, in the egg 

 of the Bird, is further consolidated by the deposition of particles of car- 

 bonate of lime in its areolae ; but it undergoes no higher organization. 

 In the Mammal, however, this new envelope (termed the Qhorion) is a 

 formation of great importance ; being the medium through which the 

 whole subsequent nutrition of the embryo is derived. This is at first 

 taken in by means of a number of villous processes, proceeding from 

 the entire surface of the Chorion, and giving it a spongy or shaggy 

 appearance ; these processes (which are composed of nucleated cells) 

 serve as absorbing radicles, which draw in the fluids afforded by the 

 parent ; and they thus make up for the early exhaustion of the small 



