No. 3-] THE VERTEBRATE HEAD. 5 19 



In Fig. 29,^ the view is taken from below ; the embryo has 

 been removed from the blastoderm and placed upon its dorsal 

 surface and the recurved edges of the neural folds are thus 

 brought prominently into view. This is of course the most 

 favorable position for making observations. The dish contain- 

 ing the embryo should be placed over a black, non-reflecting 

 background, and the embryo rotated into the most favorable 

 position with a fine artist's brush. 



In the actual specimen from which the figures were made 

 the segments showed most beautifully. They appear like a row 

 of beads running along the ventrally recurved margin, and 

 extend with great distinctness the entire length of the embryo. 

 Those in the trunk region are continuous with those in the 

 head and pass into the latter without any transition forms. 

 There is, however, some individual variation in size of the 

 neuromeres, and they are not absolutely symmetrical on the 

 right and left sides, but the significant thing is, there is uni- 

 formly the same number on each side in a given region, such 

 as the hind-brain, or the brain region as a whole. Fig. 29 

 shows the central unsegmented piece from below with three 

 segments on either side of it, occupying a part of the head- 

 folds that is directed forwards. Following the beaded edge, 

 from the head into the trunk region we find it disappear- 

 ing from view beneath the expanded walls of the gastrular 

 cavity. Viewing the same embryo from above (Fig. 28), the 

 metameric segmentation is seen to extend the entire length of 

 the embryo and, as in the earlier stages, laterally into its 

 expanded parts. The segments are so plain that they may be 

 easily counted. There seems now to be a natural landmark 

 separating the cephalic plate from the rest of the embryo; 

 this is an abrupt downward bending (/) in the medullary 

 folds which, as I have determined, lies just in front of the 

 future origin of the vagus nerve. There are eleven metameres 

 in the lateral margins of the cephalic plate, including the ones 

 embraced in this fold. 



Fig. 30 represents an older stage, in which the medullary 



1 In the process of transferring and shading, the outUnes in this figure have 

 been rendered too symmetrical, giving to it a semi-diagrammatic character. 



