CHAPTER XXVIII 

 PHYSIOLOGY OF THE EMBRYO 



1. SYNOPSIS OF SOME OF THE IMPORTANT FACTS 

 OF EMBRYONIC DEVELOPMENT 



BY the process of fertilization the fertilized egg-cell 

 divides into many cells which form a one-cell layer below 

 the egg membrane, which takes no part in this cell division. 

 By this a cavity filled with fluid, the segmentation xravity, is 

 formed in the centre of the egg. The structure thus formed 

 is called a blastula, and the one-celled layer is called the 

 ectoderm. The blastula expands by the continual increase 

 of fluid in the segmentation cavity. Below the ectoderm a 

 second layer of cells, the endoderm, is formed. The manner 

 in which the endoderm is formed varies iii different animals. 

 Finally, at a thickened portion of the blastula, a third layer, 

 the mesoderm, is formed between the ectoderm and the 

 endoderm. From the ectoderm originate the epithelial cells 

 of the skin and its glands, the nervous system, the epithelium 

 of the sense organs, and the lens. From the endoderm are 

 formed the epithelial cells of the alimentary canal and its 

 glands and those of the urinary tubes. From the mesoderm 

 originate the blood and blood vessels, the muscles, connec- 

 tive tissue, and the reproductive cells. 



The thickening of the walls of the blastula at which the 

 mesoderm originates is called the germinal disk ; in this the 

 first traces of the embryo are seen. The germinal disk 

 assumes a biscuit form, its borders curving inward and 

 thereby separating that part of the cavity of the blastula 



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