212 ILLINOIS STATE ACADEMY OF SCIENCE 



THE EARLY DEVELOPMENT OF THE VERTE- 

 BRAL COLUMN OF THE ALLIGATOR. 



George M, Higgixs, Knox College 



This study upon the early development of the verte- 

 bral column in the alligator was made upon the collec- 

 tion of vertebrate embryos in the zoological laboratories 

 of the University of Illinois. Successive stages of young 

 alligators, ranging in length from 6 mm. to 30 mm., were 

 sectioned transversely; and the attempt was made to 

 determine the origin of those mesodermal elements which 

 contribute toward the formation of the vertebra, as well 

 as their subsequent relation to each other in this for- 

 mation. Reconstructions in wax have been made of the 

 developing vertebra in the caudal, sacral, lumbar and 

 thoracic regions of the various stages; so that careful 

 comparisons could be made within these areas. This pre- 

 liminary report will concern itself with the earlier dif- 

 ferentiation only, and the more complete discussion will 

 follow in a later paper. 



In an embryo. 6 mm. long, the entire skeletal support 

 rests in the notochord, a fibrous rod-like structure which 

 lies ventral and parallel to the central nervous system. 

 At this stage those cells which comprise the chorda are 

 already migrating toward the periphery, so that a more 

 or less indefinite epitheliomorph layer may now be iden- 

 tified (Fig. 1.) As a result of this migration, large vacu- 

 oles have appeared within the chorda, some of which 

 measure fully one-third the diameter of the notochord 

 itself. Immediately external to the epitheliomorph layer 

 is a rather prominent deeply stained notochordal sheath, 

 which, in' the alligator, is not differentiated into two 

 layers as is true for the Elasmobranchs and Ichthyopsida 

 in general. In these embryos this sheath is a relatively 

 thin layer, and under a magnification of 700 diameters it 

 presents a fibrous appearance. It would further appear 

 that this notochordal sheath is probably not a product 

 of the tissue external to the chorda ; but that it may arise 

 from those cells within the notochord itself, and possi- 

 bly as a secretion from the epitheliomorph layer. 



