xviii TRANSLATORS INTRODUCTION 



a gastrula. The central cavity becomes the cavity 

 of the gut; the pore leading into it marks the 

 hind end of the future animal, in the case of verte- 

 brates, and is known as the blastopore. The layer 

 of cells lining the cavity of the sack is known as 

 the hypoblast, and gives rise chiefly to the cells 

 lining the alimentary canal of the future animal. 

 The outer layer of cells is known as the epiblast, 

 and forms the outer layer of the skin, and, along 

 the future dorsal line, gives rise to the nervous 

 system. The muscles and skeleton and the repro- 

 ductive cells arise from a set of cells known as the 

 mesoblast, that are formed chiefly from the hypo- 

 blast, and that push their way in between the 

 hypoblast and epiblast. 



This general course of development may be 

 traced in all members of the vertebrate group, and, 

 with slight modifications, may be applied to a large 

 number of invertebrates. As the modelling of the 

 general contour of the whole body and of the 

 separate organs proceeds, the protoplasm of the 

 cells gradually assumes the characters of the sub- 

 stance of muscle-cells, liver-cells, nerve-cells, blood - 

 cells, and so forth. The problem of this book will 

 become clearer if it be considered with special re- 

 ference to what goes on in these early stages. 

 Hertwig says that all the cells of the epiblast, 

 hypoblast, mesoblast, and of the later derivatives 

 of these primary layers, receive identical portions 

 of germplasm by means of doubling nuclear 

 divisions. The different positions, relations to each 

 other and to the whole organism, and to the 



