INVERSION OF THE LAYERS. 



341 



x.] 



this work; while the later changes in the embryonic 

 membranes will be dealt with in the next chapter. For 

 the changes which take place on the formation of the 

 face we may refer the reader to Fig. 113. For a full dis- 

 cussion as to the relation between the human embryos 

 just described and those of other Mammals, we refer the 

 reader to the Comp. Embryology, Vol. II. p. 224 et seq. 

 The guinea pig, rat and mouse present a pe- 

 culiar method of development, the details of which are 

 not entirely understood, and we do not propose to 

 examine them here. Suffice it to say that the mode of 

 development gives rise to the so-called inversion of the 

 layers; so called because the outer layer of the em- 

 bryonic vesicle appeared to the older observers to be 

 formed of hypoblast and the embryonic epiblast to be 

 enclosed within. 



