DEVELOPMENT OF THE EXTERNAL FORM OF THE BODY. 



135 



the trophoderm together constitute the chorion; the original cavity of the 

 blastodermic vesicle with its wall of entoderm and visceral mesoderm is the 

 yolk sac; the newly acquired cavity between the chorion and yolk sac is the 

 extraembryonic body cavity or exocoelom. The embryonic disk lies on one side 

 of, and might be said to form the roof of the yolk cavity. 



A very young human embryo described by Peters (Fig. 83) corresponds ap- 

 proximately to the stage of development shown in Fig. 90, A . The entire 



Amniotic cavity 



Muscular coat 

 of uterus 



Decidua 



parietalis + 



capsularis 



Cervix 



FIG. 118. Opened uterus containing membranes and foetus of three months. Length of 

 foetus, thirty-five millimeters. Natural size. Bonnet. 



vesicle measures about i mm. in diameter and encloses the small, flat em- 

 bryonic disk with its appended yolk sac. The disk proper consists of three 

 layers of cells the ectoderm, mesoderm and entoderm. The chorion is widely 

 separated from the yolk sac by the exoccelom. See also Fig. 106. 



An embryo slightly more advanced than that described by Peters has been 

 described by von Spec (Fig. 84) . In this case a furrow the neural groove 

 appears on the dorsal (ectodermal) side of the embryonic disk, and the latter is 



