658 MANUAL OF PHYSIOLOGY. 



three definite layers, which are called respectively, from their 

 position in the blastoderm, the epiblast, the mesoblast, and the 

 hypoblast. 



From these layers are developed the embryo and the membranes 

 surrounding it, each layer being developed into certain tissues 

 and forming these only. 



Thus from the epiblast, or outer layer, arise the epidermis ot 

 the skin, the brain and spinal cord, and certain parts of the 

 organs of special sense ; whilst it also aids in the formation of the 

 chorion and the amnion. From the mesoblast are developed the 

 skeleton, the connective tissues, the muscles, the nerves, in ad- 

 dition to the vascular system and the supporting tissue of the 



FIG. 251. 



cfc 



Transverse section of the medullary groove, and half the blastoderm of a 

 chick of eighteen hours. (Foster and Balfour.) A. Epiblast. B. Meso- 

 blast. c. Hypoblast. mf. Medullary fold. me. Medullary groove, eh. 

 Notochord. 



glands ; one kind of tessellated cells arise from this layer, viz., 

 the endothelium, forming the surface of all serous membranes. 

 From the hypoblast springs the epithelial lining of the alimen- 

 tary canal, of the glands which are diverticula from it, and of 

 the lungs ; it also forms the lining membrane of the allantois and 

 yolk sac. 



The blastoderm of the hen's ovum, which is comparatively 

 easily studied, consists of a small clear central portion, called the 

 area pellucida, from which the body of the chick arises. Sur- 

 rounding the area pellucida is a much larger zone, which appears 

 less transparent ; this, the area opaca, is devoted to the formation 

 of the membranes. 



The embryo is developed from the rest of the blastoderm in 



