284 ANIMAL PHYSIOLOGY. 



wise takes place, and, for a period, two separate spaces 

 exist within the uterus, namely, that of the general 

 uterine cavity, and that which contains the ovum. It is 

 some considerable time before this latter space grows suffi- 

 ciently for the decidua vera and decidua reflexa to come into 

 contact. Ultimately they are so closely blended that in the 

 later months of gestation the decidua reflexa can no longer 

 be recognised. 



While the ovular space is separated, as has been said, from 

 the uterine space by the decidua reflexa, it remains from the 

 first in contact with the uterine wall on one side; and the 

 mucous membrane of the uterus at this part exhibits, like 

 the rest, an exaggerated growth, sometimes termed decidua 

 serotina, destined to be more highly developed than the 

 decidua vera, and becomes the medium of connection between 

 the maternal structures and the child, after the vessels in 

 the villi of the chorion at other parts have disappeared. The 

 vessels referred to in the villi of the chorion are brought to 

 it on the surface of a vesicular outgrowth of the embryo, 

 named the allantois (p. 294); and, where in, contact with the 

 decidua serotina, the vessels of this allantois become greatly 

 developed, as well as those of the uterine mucous membrane, 

 and a structure is developed, called the placenta or after- 

 birth, by means of which the formed embryo orfcetus receives 

 nourishment from the mother till oirth. 



207. The Embryo. We have seen that after impregna- 

 tion the contents of the ovum are converted, by the disap- 

 pearance of the spermatozoa and germinal vesicle, and the 

 cleavage of the yelk, into a mass of nucleated corpuscles. 

 This happens in the lower part of the Fallopian tube, and 

 thereafter the interior of the yelk becomes transparent, and 

 the nucleated corpuscles are aggregated beneath the zona 

 pellucida in the form of a hollow sphere, which is termed the 

 blastoderm or germinal membrane. Within a few days after 

 the ovum has reached the uterus, a clear area, with an opaque 

 border, makes its appearance on one side of the germinal 

 membrane, and in this area a white streak, which is the first 

 appearance of the embryo. This streak consists of a furrow 

 with elevated margins, the primitive groove; and beneath 

 the groove a rod-like body soon appears, the chorda dorsalis 



