134 HAY. [VOL. I. 



about nine sections. The emission of cells begins at about 

 the fourth section from the front, is seen most distinctly in the 

 sixth, and is not seen in the seventh. Hence, the cells are 

 given off from about the middle third of the upper edge 

 of the protovertebra. A string of these cells can be followed 

 over the spinal cord, where they join those from the other side. 

 They lie above other cells which have come from the ventral 

 border of the protovertebral plate. I have nowhere found any 

 evidences that cells derived from the upper edge of the proto- 

 vertebral plate grow downward toward the notochord. The 

 sections represented on page 181 of Gadow and Abbott's paper 

 are, in my opinion, in too advanced a stage to reveal clearly 

 the origin of the skeletogenous cells. The evidence of this is 

 to be found in the great number of cells which crowd the space 

 above the spinal cord. These have undoubtedly come from 

 both the ventral and dorsal borders of the protovertebrae. In 

 the sections which I have examined the two sets of cells are 

 caught before they have commingled. 



It is to be noted that, according to the authors, all the inter- 

 digitating dorsal and ventral masses of cells are formed during 

 the development of the skeletogenous tissue. Also, the colo- 

 nies of cells are planted which give rise to the interdorsals and 

 the interventrals. Furthermore, each ventral sklerotome fuses 

 with the dorsal sklerotome of the succeeding protovertebra to 

 form a skleromere. But a stage at once ensues in which every 

 trace of segmentation is lost in the sheath of cells surrounding 

 the notochord, and " metamerism is then shown only by the 

 nerves and by the cavities in the dorsal portion of the myo- 

 tomes." This being true, it would be interesting to learn in 

 what way the fusion of a ventral sklerotome with the dorsal 

 of the next segment differs from its fusion with the dorsal of 

 its own segment. 



But the cells which constitute the dorsal and the ventral 

 sklerotomes later give origin to the cartilaginous pieces, eight 

 in number, which are called basalia and interbasalia. How, in 

 view of the fact that in one stage all the boundaries between 

 the sklerotomes disappear, we are to know that the base of the 

 ventral sklerotome is converted into the basiventral cartilage, 



