496 



HISTOLOGY 



mammal, where the young start off with practically no yolk supply and 

 depend entirely upon materials from the parent's blood for everything. 

 We shall describe briefly this uterine surface together with the embryonic 

 membranes by which the young animal is able to take advantage of these 

 supplies. 



The normal adult uterus, in many mammals, consists of an outer 

 muscular layer with an inner mucous layer that is composed of a thick 

 subepithelial connective tissue containing many lymph spaces and 

 blood vessels. The epithelium which lines the mucous layer is a simple 



FIG. 465. Part of a longitudinal section through the uterine wall of a cat. The foetal mem- 

 branes of a very young embryo are seen in situ., applied to it. am., amnion; ch., chorion; 

 bl., blood. Dotted lines show region from which next section was drawn, x 45. 



columnar epithelium which is invaginated into a great many simple, or 

 sometimes branched, tubular glands that reach to the muscle layer and 

 secrete no special fluid. The outer part of this layer with its epithelium 

 and part of the glands are broken up and thrown off at a period called 

 the menstrual period. This lasts a few days, after which the lost epi- 

 thelium is regenerated from the remaining portions of the glands. 



The fcetal membrane is composed of two layers, an outer layer cov- 

 ered externally with a simple epithelium, under which is some embryonic 

 mesodermal tissue. This is the chorion. Also an inner layer composed 

 likewise of a simple, but inner, epithelium resting on a mesodermal 

 layer. This is called the amnion. These two layers are joined together 

 by their mesodermal surfaces, the line of contact being marked by a very 



