400 



an older stage than that represented in fig. 2. The medullary folds 

 are now fully formed and their ventral curvature is very marked. 

 In fig. 3, the view is taken from below. The embryo has been 

 removed from the blastoderm and placed upon its dorsal surface. 

 This is, of course, the most favorable point of view for making ob- 

 servations; the glass dish containing the embryo, should be placed 

 voer a black, non-reflecting, background and the embryo rotated into 



Fig. 3 



Figs. 3 and 4. Represent two views of the same embryo after the formation of the 

 medullary folds. The folds are ventrally curved. Fig. 3 view from below, Fig. 4 from 

 above. 1 — 11, segments in front of the future vagus nerve, op optic vesicle, yc gastrular 

 cavity. 



the most favorable position with a fine artist's brush. In the actual 

 specimen from which this figure was 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 con- 

 tinuous with those in the head, and pass into the latter without any 

 transition forms. The segments may be easily and surely counted. 

 I wish to call especial attention to the position of the three anterior 

 ones: they occupy that part of the medullary fold that is directed 



