No. 3.] THE VERTEBRAL COLUMN OF FISHES. 133 



Considering too that this discovery is of so much importance, 

 it is unfortunate that the authors did not illustrate their paper 

 with better and more convincing figures. Several text figures 

 representing cross sections are given on page 181 ; but they are 

 hardly satisfactory. Taking into consideration the curvature 

 of the protovertebral plates it must be exceedingly difficult, in 

 cross sections, to distinguish the limits between two such inter- 

 digitating masses of cells. One would suppose that sagittal 

 sections would furnish better evidences, but the authors have 

 not given such a section through the proper portion of the 

 tail. 



It has been my fortune to be permitted to examine several 

 well-prepared series of embryos of Squalus acantJiias ; and for 

 this opportunity I am indebted to Dr. VV. M. Wheeler, of the 

 University of Chicago. One series consists of sections of a 

 specimen only 5 mm. in length ; two other series belonged 

 to embryos 6 and 7 mm. long. I have studied these with the 

 view of testing the statements of the authors referred to, and 

 I trust that I have done this without prejudice. I know no 

 reason for objecting to the theories of the learned authors, 

 in case they are well founded. 



In the tail of the 5 mm. Sqnahis cells from the lower border 

 of the protovertebra are seen to be given off toward the noto- 

 chord. They lie against the latter on each side, and the 

 cluster extends above the notochord so as to reach the lower 

 border of the spinal cord. More anteriorly the skeletogenous 

 cells rise still higher. It was observed that, as the interproto- 

 vertebral spaces are approached, the cells appear to drop to a 

 lower level, and may then rise not higher than the middle 

 of the notochord; but this condition lasts for only the thick- 

 ness of about two sections. It seems to me possible that this 

 may furnish an explanation of some of the appearances pre- 

 sented in the text figures given by the authors on page 181. 



In no sections of specimens 5 mm. and 6 mm. long did I 

 find any indications of the emission of cells from the upper 

 ends of the protovertebrae. This was first seen in sections of 

 the embryo 7 mm. long and at about the thirtieth segment 

 behind the head. In this series each body segment comprises 



