844 



IMPREGNATION. 



inner layer, or true amnion, is continuous with the skin of the embryo at the 

 umbilicus, and closely envelops it. The amniotic cavity or sac thus formed 

 becomes filled with the liquor amnii, which gradually increases in quantity 

 as pregnancy advances, up to about the fifth or sixth month, when the 

 quantity gradually decreases up to the time of labor. 



FIG. 221. 



FIG. 222. 



a, chorion with villi. The villi are shown to be best developed in the part of the chorion to 

 which the allantois is extending ; this portion ultimately becomes the placenta, b, space between 

 the two layers of the amnion ; c, amniotic cavity ; d, situation of the intestine, showing its con- 

 nection with the umbilical vesicle ; e, umbilical vesicle ;/, situation of heart and vessels ; g, allan- 

 tois. (After Todd and Bowman.) 



787. At about the time of the commencement of the development of 

 the amnion a new organ, the allantois, appears as a pyriform mass of cells at 

 a point immediately posterior to the vitelliue duct and projecting through 

 the same opening. (Fig. 223.) This mass of cells undergoes rapid growth, 

 spreading itself between the true and false amniotic folds, finally completely 



FIG. 223. 



FIG. 224. 



FIG. 223. Diagram of Fecundated Egg. (After Dalton.) a, umbilical vesicle ; 6, amniotic 

 cavity ; c, allantois. 



FIG. 224. Fecundated Egg with Allantois nearly Complete, a, inner layer of amniotic fold ; 

 6. outer layer of ditto ; c, point where the amniotic folds come in contact. The allantois is seen 

 penetrating between the outer and inner layers of the amniotic folds. This figure, which repre- 

 sents only the amniotic folds and the parts within them, should be compared with Figs. 221 and 

 222, in which will be found the structures external to these folds. 



enclosing the embryo and amnion (Fig. 224), becoming at the same time 

 adjoined to the false amnion, when it is developed into the true chorion. Dur- 

 ing the process of the development of the allantois, it has become very vas- 

 cular ; at first there are two arteries and two veins, afterward one of the 

 veins disappears. These vessels constitute the umbilical vessels, forming part 

 of the umbilical cord which connects the allantois with the embryo. During 



