52 THE FIRST DAY. [CHAP. 



give rise to the opacity of that part, which we have 

 spoken of as the embryonic shield. 



At the edge of the area pellucida the hypoblast 

 becomes continuous with a thickened rim of material, 

 underlying the epiblast, and derived from the original 

 thickened edge of the blastoderm and the subjacent 

 yolk. It is mainly formed of yolk granules, with a 

 varying number of cells and nuclei imbedded in it. It 

 is known as the germinal wall, and is spoken of more in 

 detail on pp. 65 and 66. 



The epiblast is the Hornblatt (corneal layer), and the hypo- 

 blast the Darmdrusenblatt (epithelial glandular layer) of the 

 Germans, while those parts of the mesoblast which take part in 

 the formation of the somatopleure and splanchnopleure cor- 

 respond respectively to the Haut-musJcel-platte and Darm-faser- 

 platte. 



All blood-vessels arise in the mesoblast. Hence the vascular 

 layer of the older writers falls entirely within the mesoblast. 



The serous layer of the old authors includes the whole of 

 the epiblast, but also comprises a certain portion of mesoblast ; 

 for they speak of all the organs of animal life (skin, bones, 

 muscle, &c.) as being formed out of the serous layer, whereas the 

 epiblast proper gives rise only to the epidermis and to certain 

 parts of the nervous system. In the same way their mucous layer 

 corresponds to the hypoblast with so much of the mesoblast as 

 takes part in the formation of the organs of organic life. Their 

 vascular layer therefore answers to a part only of the mesoblast 

 viz. that part in which blood-vessels are especially developed. 



From the 8th to the 12th hour. The changes 

 which next take place result in the complete differen- 

 tiation of the embryonic layers, a process which is inti- 

 mately connected with the formation of a structure known 

 as the primitive streak. The full meaning of the 



