814 



DEVELOPMENT. 



[CH. LVIII. 



other terms that are employed. The part of the primitive 

 alimentary canal enclosed by the head-fold is called the fore-gut ; 

 that enclosed by the tail-fold is called the hind-gut ; the remainder 

 is called the mid-gut. 



We have now seen the way in which a distinct embryo with 

 foreshadowings of the future organs is formed. In subsequent 



Fig, 629. Diagrammatic section showing the relation in a mammal between the primitive 

 alimentary canal and the membranes of the ovum. The stage represented in this 

 diagram corresponds to that of the fifteenth or seventeenth day in the human embryo, 

 previous to the expansion of the allantois ; c, the villous chorion ; a, the amnion ; a', 

 the place of convergence of the amnion and reflexion of the false amnion a" a", or 

 outer or corneous layer ; e, the head and trunk of the embryo, comprising the primitive 

 vertebrae and cerebro-spinal axis ; i, i, the simple alimentary canal in its upper and 

 lower portions. Immediately beneath the right hand i is seen the foetal heart, lying 

 in the anterior part of the pleuro-peritoneal cavity ; v, the yolk-sac or umbilical vesicle ; 

 vi, the vitello-intestinal opening ; u, the allantois connected by a pedicle with the 

 hinder portion of the alimentary canal. (Quain.) 



sections we shall have to study the way in which each set of 

 organs is elaborated from these primitive structures. We may 

 conclude this section by giving a list of the organs which are 

 formed from the several primary embryonic layers : 



i. From Epiblast a. The epidermis and its appendages. 



b. The nervous system, both central and peripheral. 



c. The epithelial structures of the sense-organs. 



