192 EMBRYOLOGY. 



(d) The place where blood-vessels and connective 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 : 

 First by the area pellucida, 



Secondly by the va^ular area ending in the sinus- 

 term inalis, 



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 organ* 

 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- 

 lens. 



II. Primary Inner Germ-layer. 



1. Entoblast, or secondary inner germ-layer. 



Epithelium of the alimentary canal and its glands, epitheliums 

 of urinary bladder. 



2. Fundament of the chorda. 



3. The middle germ-layers. 



A. Primitive 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. Mesenchyme- germ. 



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

 lymphoid organs, smooth involuntary muscles. 



