PART I. 



SYSTEMATIC EMBRYOLOGY. 



INTRODUCTION. 



IN all the Metazoa the segmentation is followed by a series 

 of changes which result in the grouping of the embryonic cells 

 into definite layers, or membranes, known as the germinal 

 layers. There are always two of these layers, known as the 

 epiblast and hypoblast; and in the majority of instances 

 a third layer, known as the mesoblast, becomes interposed 

 between them. It is by the further differentiation of the ger- 

 minal layers that the organs of the adult become built up. 

 Owing to this it is usual, in the language of Embryology, to 

 speak of the organs as derived from such or such a germinal 

 layer. 



At the close of the section of this work devoted to systematic 

 embryology, there is a discussion of the difficult questions which 

 arise as to the complete or partial homology of these layers 

 throughout the Metazoa, and as to the meaning to be attached 

 to the various processes by which they take their origin ; but a 

 few words as to the general fate of the layers, and the general 

 nature of the processes by which they are formed, will not be 

 out of place here. 



Of the three layers the epiblast and hypoblast are to be 

 regarded as the primary. The epiblast is essentially the primi- 

 tive integument, and constitutes the protective and sensory 

 layer. It gives rise to the skin, cuticle, nervous system, and 

 organs of special sense. The hypoblast is essentially the diges- 

 tive and secretory layer, and gives rise to the epithelium lining 

 the alimentary tract and the glands connected with it. 



