No. 3-] 



THE VERTEBRATE HEAD. 



517 



they extend also, with less distinctness, into the trunk and fade 

 away without reaching the hinder extremity. The furrows do 

 not show so distinctly in cross-section as one would at first 

 suppose, and they are in part an optical effect, arising from the 

 way in which the neural folds are formed. This will be under- 

 stood on reference to cut 5. There is, nevertheless, a distinct 

 notch to be seen in the cross-sections of many specimens, while 

 in others it is lacking. I am inclined, with my present light, 



Cut 4. — Embryo of Acanthias just after the formation of the neural folds, x about 40 diameters. 

 Metameres not shown. The transverse lines indicate the plane of the sections in cut 5. 



to consider these furrows as purely mechanical effects. Sections 

 show (cut 3) that the marginal bands, at a stage just younger 

 than this one, are composed of an accumulation of cells form- 

 ing thickened cords running along the margins of the embryo. 

 These bundles of cells and their immediate derivatives are the 

 material out of which the neural ridges and a large part of the 

 medullary folds are straightway produced. 



As noted in the preceding embryo, the metameres are most 

 clearly seen from below, but the reason for this is not far to 



