No. 3.] 



THE VERTEBRATE HEAD. 



515 



rowed in front by folding in of the sides, so that the embryo, 

 when viewed directly from in front, seems mounted on a keel; 

 the keel, however, is not 

 solid, but contains the an- 

 terior part of the gastrular 

 cavity, which still reaches to 

 the anterior limit of the head. 

 For the purpose of fixing the 

 stages as definitely as pos- 

 sible, I give anatomical char- 

 acteristics not immediately 

 connected with the meta- 

 meric segmentation. The 

 embryo from which the figure 

 was made measured if*o ^im. 

 in length, but there is so 

 much individual variation in 

 the size of embryos of apparently the same age, that the length 

 is not very significant. There are, in that part of the embryo 



Cut I. — Embryo and blastoderm of Acanthias 

 just before the formation of the neural folds. 

 X about 8 diameters. 



Cut 2. — The same embryo x about 40 diameters. Metameres not shown. The transverse 

 lines and numbers indicate the plane of the sections shown in the succeeding cut. 



behind the cephalic region, three or four mesodermic somites 

 rather imperfectly differentiated. The metameric segmentation 



