300 



HISTOLOGY. 



condition of the mucosa of the human uterus when the fertilized ovum enters 

 it is unknown. The stage of development of many young human embryos sug- 

 gests that their growth began nearer the time of the first menstruation which 

 lapsed than the last which occurred. This may be due to the frequency of 

 human menstruation, which may still be preparatory to coitus as in other 

 mammals. 



THE DEVELOPMENT OF THE DECIDUAL MEMBRANES. 

 Before describing the mucosa of the uterus during pregnancy, it is 

 necessary to consider the membranes of the embryo which are in contact 

 with it. Fig. 343, A, represents a blastodermic vesicle in which the three 

 germ layers are present. (The formation of such a vesicle by the seg- 

 mentation of the ovum has been figured on page 19.) In a thickened 

 portion of the outer layer of the vesicle a cleft occurs, which in B has 

 widened and become the amniotic cavity. It is bounded below by the 



A 



FIG. 343. THREE DIAGRAMS OF THE HYPOTHETICAL DEVELOPMENT OP THE HUMAN 



DECIDUAL MEMBRANES. (After Minot.) 



al., Allantois; am., amnion; am. c., amniotic cavity; cho., chorion; coe., coelom; y. s., yolk sac. The 

 mesoderm is stippled, the ectoderm is shaded with lines and the entoderm with dots. 



ectoderm which covers the body of the embryo, and above by a layer 

 which is soon divided into two parts by an extension of the body cavity. 

 This has occurred in C. The inner layer or amnion consists of ectoderm 

 toward the embryo and mesoderm away from it. It is a membrane con- 

 tinuous with the skin of the embryo. The outer layer or chorion surrounds 

 the entire vesicle and is characterized by shaggy villi. It consists of ecto- 

 derm [trophoblast] on its peripheral surface and mesoderm within. 

 A stalk of mesenchymal tissue surrounding the allantois extends from the 

 embryo to the chorion. It lodges the umbilical (allantoic) vessels through 

 which the blood of the embryo passes to the chorionic villi and back to 

 the embryo. These villi enter into close relation with uterine mucosa, 

 being bathed in maternal blood, and the embryo receives such nutriment 

 as is absorbed through their walls. Human embryos of the stage C are 



