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 dif- 

 ferentiation of the germinal 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 primitive 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 digestive and secretory layer, and gives 

 rise to the epithelium lining the alimentary tract and the glands 

 connected with it. 



The mesoblast is only found in a fully developed condition in the 

 forms more highly organized than the Ccelenterata. It gives origin 

 to the general connective tissue, internal skeleton, the muscular 

 system, the lining of the body-cavity, the vascular, and excretory 

 systems. It probably in the first instance originated from dif- 



