THE FCETAL MEMBRANES OF MAMMALS. 



225 



sole-like form; meanwhile it grows to a length of about 3-5 mm. 

 (fig. 131). 



As has been already described in the previous chapter, at this 

 time the middle germ-layer spreads out in the embryonic fundament, 

 the medullary groove (figs. 130 and 131 rf), the chorda, and a, 

 number of primitive segments are formed, 

 and, on the eighth day, the first trace of 

 the vessels and blood appears in the vas- 

 cular area (o). On the ninth and tenth 

 days the embryonic fundament is by a 

 process of folding converted into the body 

 of the embryo, and is constricted off from 

 the remaining part of the blastodermic 

 vesicle, out of which at the same time 

 various foetal membranes begin to be de- 

 veloped. The initial stages of all these 

 processes are the same in Mammals as in 

 Birds and Eeptiles, so that we can express 

 ourselves very briefly in describing them. 

 We shall connect the description with the 

 diagrammatic drawings which KOLLIKEB 

 has made, and which have found a place 

 in many text-books (fig. 132, 1-5). 



Diagram 1 shows a blastodermic vesicle 

 which in the Rabbit would correspond to 

 about the seventh or eighth day. It is 

 still enclosed from without by the very 

 much attenuated vitelline membrane (d), 

 which is now also called prochorion, since 

 in many Mammals flakes and shreds of 

 albumen have been precipitated on its 

 outer surface out of the fluid secreted by 

 the mucous membrane of the uterus. The 

 inner germ-layer (i) which in a slightly 



younger blastula, such as is represented in figure 62 J5, reaches only 

 to the line ge, and still leaves uncovered a third of the inner surface 

 of the sphere has now entirely grown around to the vegetative pole. 

 The middle germ-layer (m) is in full process of development, and 

 embraces about a fourth part of the surface of the sphere. A small 

 portion of this three-layered region contains the embryonic fundament, 

 which would be in about that stage of development which we have 



15 



Fig. 131. Embryonic fundament 

 of a Rabbit of nine days -with 

 a portion of the area pellucida, 

 from KOLLIKER. 



Ap, Area pellucida ; ao, area opaca ; 

 h', h", h'", medullary plate in 

 the region of the first, second, 

 and third cerebral vesicles ; stz t 

 stem-zone (Stammzone) ; pz, 

 parietal zone ; )/, dorsal furrow - t 

 pr, primitive streak. 



