192 EMBRYOLOGY. 



(d) The place where blood-vessels and coniiective substance at 

 first arise in the opaque area is sharply limited at the 

 periphery by a circular vessel, the sinus terminalis. 

 ■(e) Since the outer and the inner germ-layers further con- 

 tinue to spread themselves out over the yolk after the 

 development of the intermediate layer, the body of the 

 embryo becomes surrounded by three areas : — 

 Pirst by the area pellucida, 

 Secondly by the vascular area ending in the sinus 



terminalis, 

 Thirdly by the yolk-area, which is coextensive with 

 the margin of the overgrowth. 

 '8. The red blood-corpuscles of all Vertebrates possess in the 

 •earliest stages of development the power of increase by means 

 of division. The red blood-corpuscles of Mammals have at this 

 time a nucleus, 



9. The following table gives a survey of the fundamental organs 

 •of the embryo, and the products of their further development : — 



I. Outer Germ-layer. 



Epidermis, hair, nails, epithelium of dermal glands, central nervous 

 system, peripheral nervous system, epithelium of sensory organs, the 

 Jens. 



II. Primary Inner Germ-layer. 



1. Entohlaat, or secondary inner germ-layer. 



Epithelium of the alimentary canal and its glands, epithelium 

 of urinary bladder. 



2. Fundament of the chorda. 



3. Tlie middle germ-layers. 



A. Prvmitive Segments. 

 Transversely striped, voluntary muscles of the body. Parts 

 of the mesenchyme. 



B. Lateral Plates. 

 Epithelium of the pleuroperitoneal cavities, the sexual cells 

 and epithelial components of the sexual glands and their 

 outlets, epithelium of kidney and ureters. Parts of the 

 mesenchyme. 



4. Mesenohyme-germ, 



Group of the connective substances, blood-vessels and blood, 

 lymphoid organs, smooth involuntary muscles. 



