46 PRINCIPLES OF PHYSIOLOGY 



portions. One part is called the trophoblast. 

 It has to do with the formation of structures 

 for connecting the ovum with the mucous 

 layer of the uterus in which the embryo is 

 to spend the first part of its existence. The 

 other is the blastoderm, in which the future 

 being is to be developed. The blastoderm 

 divides into three germinal layers, an inner 

 called the endoderm, an outer named the 

 ectoderm, and between the two a third, the 

 mesoderm. The embryo at first consists of 

 only two layers, the endo- and ecto-derm, but 

 at a very early period the mesoderm makes its 

 appearance and is probably formed by the 

 other two. In turn, the mesoderm splits 

 into two layers, one of which becomes closely 

 adapted to the ectoderm, to form a thick layer, 

 the somatppleure, while the other clings to the 

 endoderm and becomes the splanchnopleure. 

 The somatopleure becomes the wall of the body, 

 and the splanchnopleure forms the outer wall 

 of the alimentary canal. Between the two 

 there is a space, the body cavity, which, in 

 full development, constitutes the pleural and 

 peritoneal cavities, in which lie the viscera of 



