REPRODUCTION. 



Derivatives from the Layers. 



855 



Ectoderm, or Epiblast. From the epiblast are developed the 

 central nervous system and the epidermal tissues. 



Mesoderm, or Mesoblast. From the mesoblast arise most of the 

 organs of the body. These include the vascular, muscular, and 

 skeletal systems; also the generative and excretory organs; but not 

 the bladder, the first part of the male urethra, nor the female urethra. 



Entoderm, or Hypoblast. The hypoblast is the secretory layer. 

 From it spring the intestinal epithelium and that of the glands 



Fig. 390. Diagram of an Early Stage of a Primate Embryo. ( MINOT. ) 



All, Allantois. Am, Amnion. b.s., Body-stalk. Cho, Chorion. Emb, Embryo. 

 In, Entodermal Cavity of Embryo, vi, Villi of Chorion. yk, Yelk-sac. 



which open into the intestines; also the epithelum of the resipra- 

 tory system, the bladder, the prostatic part of the male urethra, and 

 the entire female urethra. 



Up to this point the cavity of the germ is one undivided com- 

 partment bounded by splanchnopleure. By infolding of the splanch- 

 nopleure this cavity is divided into two smaller compartments of 

 unequal size. The smaller is the gut-tract; the larger, the yelk-sac, 

 or umbilical vesicle. The communication between the two cavities is 

 the vitellim duct. 



With the unfolding of the splanchnopleure the somatopleure 

 also follows, to form the body-walls of the embryo. Part of the 



