218 ILLINOIS STATE ACADEMY OF SCIENCE 



the inner layer; but are composed entirely of the larger 

 spherical cells. 



By a further proliferation of the cells of the membran- 

 ous neural arches, the origin of which occurred in the 

 earlier stage, and by a further addition of cells from 

 the myotome, the entire spinal cord is at this 10 mm. 

 stage covered dorsally and laterally by a continuous 

 membranous structure. Openings, of course, occur for 

 the exit of the spinal nerve roots and for the inclusion, 

 of the dorsal ganglion within each segment. This struc- 

 ture, although continuous, is much thinner in the inter- 

 vertebral regions, since the cartilaginous vertebrae are 

 to arise in the position of the original sclerotomes. 



The first appearance of a cartilage vertebra may be 

 identified in this same 10 mm. embryo. In the approxi- 

 mate position of the original caudal part-sclerotomes of 

 the trunk region, the cells of the membranous sclerotome 

 have transformed into a precartilage neural arch which 

 rests upon the inner layer of the primary centrum. This 

 precartilage neural arch extends dorsally and slightly 

 posteriorly, and terminates just back of the upper mar- 

 gin of the dorsal ganglion. The lower or haemal arches 

 appear as mere stumps in the trunk region, but in the 

 tail these structures are much longer than before and 

 nearly encircle the caudal blood vessels. However in 

 neither region is there any evidence of a precartilage 

 formation within these lower processes. N"or has such 

 a transformation yet occurred within the primary cen- 

 trum. So that the vertebral column of a 10 mm. alligator 

 may be said to be composed of a series of independent 

 precartilage neural arch processes which rest upon the 

 membranous primary centrum and lie along the lateral 

 aspect of the spinal cord. 



A further consideration of the later development of 

 the column will appear in a subsequent paper. 



