THE LAW OF EVOLUTION CONTINUED. 379 



cells, vaguely distinguished from the internal ones by their 

 minuter sub-division as well as by their greater complete- 

 ness, coalesce to form the blastoderm or germinal mem- 

 brane. Presently, one portion of this membrane is ren- 

 dered unlike the rest by the accumulation of cells still 

 more sub-divided, which, together, form an opaque roundish 

 spot. This area germinaliva, as it is called, shades off 

 gradually into the surrounding parts of the blastoderm ; and 

 the area pellucida, subsequently formed in the midst of it, 

 is similarly without precise margin. The " primitive trace," 

 which makes its appearance in the centre of the area pellu- 

 cida, and is the rudiment of that vertebrate axis which is to 

 be the fundamental characteristic of the mature animal, is 

 shown by its name to be at first indefinite — a mere trace. 

 Beginning as a shallow groove, it becomes slowly more pro- 

 nounced: its sides grow higher; their summits overlap, 

 and at last unite; and so the indefinite groove passes into a 

 definite tube, forming the vertebral canal. In this vertebral 

 canal the leading divisions of the brain are at first discern- 

 ible only as slight bulgings; while the vertebrae commence 

 as indistinct modifications of the tissue bounding the canal. 

 Simultaneously, the outer surface of the blastoderm has 

 been differentiating from the inner surface : there has arisen 

 a division into the serous and mucous layers — a division 

 at the outset indistinct, and traceable only about the germi- 

 nal area, but which insensibly spreads throughout nearly the 

 whole germinal membrane, and becomes definite. From 

 the mucous layer, the development of the alimentary canal 

 proceeds as that of the vertebral canal does from the serous 

 layer. Originally a smple channel along the under sur- 

 face of the embryonic mass, the intestine is rendered dis- 

 tinct by the bending down, on each side, of ridges which 

 finally join to form a tube — the permanent absorbing sur- 

 face is by degrees cut off from that temporary absorbing 

 surface with which it was continuous and uniform. And in 

 an analogous manner the entire embryo, which at first lies 



