656 



MANUAL OF PHYSIOLOGY. 



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 



yelk sack. 



FIG. 251. 



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

 eighteen hours. (Foster and Balfour.) A. Epiblast. B. Mesoblast. c. Hypoblast. mf. 

 Medullary fold. me. Medullary groove. ch. Notochord. 



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 



FIG. 252. 



N.C. 



F.So. 



Diagrammatic longitudinal section through the axis of an embryo chick. (Foster and 

 Balfour.) N.C. Neural canal. Ch. Notochord. D. Fore gut. F.So. Somatopleure. F.Sp. 

 Splanchnopleure. Sp. Splanchnopleure forming the lower wall of the fore gut. Ht. 

 Heart, pp. Pleuroperitoneal cavity. Am. Amniotic fold. A. Epiblast. B. Mesoblast. 

 C. Hypoblast. 



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

 of the membranes. 



The embryo is developed from the rest of the blastoderm in 

 the following manner : At the front of the area pellucida a fold, 



