130 THE DEVELOPMENT OP 



From our historical review it will be seen that the 

 great majority of the evidence is in favor of the follow- 

 ing points : (1) The segmentation begins at the centre 

 of the egg. (2) The blastoderm is formed by the migra- 

 tion of the cells produced by segmentation to the surface. 

 (3) The endoderm, in many types of hexapods is formed 

 by cells which remain behind in the yolk. (The exceptions 

 to this will be considered later.) To these a fourth is to 

 be added, which is so generally recognized as to need but 

 little argument : The primitive groove of hexapods is the 

 homologue of the blastopore. This is shown by its rela- 

 tions to the origin of the mesoderm and, later, of the nervous 

 system, which are almost exactly like those of verte- 

 brates, 1 where the same homology is recognized. Except 

 in a few cases in the hexapods (Korotneif, Patten, Bruce, 

 etc.), it is not claimed that the primitive groove is in any 

 way connected with the actual production of the endoderm 

 or that there is any passage of cells from the blastoderm 

 to the interior of the egg. How then has this state of 

 affairs arisen? and how is it to be explained? 



In all these there is a migration to the surface closely 

 like that of Crangon. Now, in this genus (and the same 

 is apparently true of other forms, e. g., Atyephyra) the 

 majority of these migrating cells go to form the germinal 

 area, more going to that region than to any other, and, as 

 fig. 9 shows, the later migration is all toward that point. 

 Here it is that the gastrula is formed, and as a necessary 

 result some of these very cells are returned by that oper- 

 ation into the yolk from which they have just emerged. 

 Should some of these migrant cells be still farther de- 

 layed, it is a plausible supposition that they might be- 



1 As will be shown later, the supra-cesophageal commissure, which completes 

 the ojsophageal ring and thus makes the invertebrate nervous system so different 

 ironi that of the vertebrate, is developed alter the rest of the system is outlined. 



