PAPERS OX BIOLOGY AND AGRICULTURE 213 



Those cells, which are potentially scleroblastic and 

 later contribute toward the development of the mem- 

 branons vertebrae, are derivatives of the median wall 

 of the lateral myotomes,^ which lie between the spinal 

 cord and the external body wall. Unfortunately, I have 

 been unable to secure any sufficiently early stages which 

 show the migration of these scleroblastic cells; but, fol- 

 lowing Schauinsland (1906) in his study of the very 

 early stages in the development of the vertebral column 

 of Sphenodon, the Australian lizard, it would appear 

 that all of the cells of the lower half of the medial plate 

 of each myotome are potentially scleroblastic. During 

 the process of the migration these cells come to be ar- 

 ranged into very definite groups which occupy charac- 

 teristic positions along the notochord and the spinal cord 

 (Fig. 2.) 



In the earliest embryo of my series, these groups or 

 masses of scleroblastic cells are definitely located in po- 

 sitions along the dorso-lateral and ventro-lateral margins 

 of the notochord. These groups may be readily recog- 

 nized by the large size of their constituent cells and like- 

 wise of their nuclei, as well as by the deeply staining 

 quality of the cytoplasm. It is quite possible, in this 

 stage, to recognize eight such cell groups within each 

 body segment, all of which are concerned in the develop- 

 ment of the definitive membranous vertebra. Of these 

 vertebral elements, four lie upon each side of the noto- 

 chord, two in the angle between the notochord and the 

 spinal cord and two along the latero-ventral margin of 

 the chorda. (Fig. 3.) 



Since the segmental blood vessels, which course up- 

 ward from the dorsal aorta within the dissepiments 

 between successive somites, atford an excellent means 

 for the identification of the limits of each body segment, 

 it is possible to designate these eight vertebral elements 

 by such terms as characterize their position in the seg- 

 ment. Accordingly, upon that basis, those elements 

 which lie more anterior in each segment may be desig- 

 nated as cranial and those most posterior as caudal. 

 Furthermore that element which is cranial and lies along 

 the upper margin of the notochord may be called a 



