216 ILLINOIS STATE ACADEMY OF SCIENCE 



tion is as clearly the beginning of the membranous hae- 

 mal arch ; while the larger intermediate area, a probable 

 derivative of the original cranial element, will contribute 

 largely toward the primary centrum of the vertebra. 

 At this stage, also, a rapid cell proliferation has occurred 

 between the bases of the haemal processes along the 

 ventral surface of the notochord and form a series of 

 hypochordal bars which are to form the lower portions 

 of the primary centra. (Fig. 5.) 



It is very evident that the scleroblastic cells compris- 

 ing the sclerotome are clearly of two kinds. Those cells 

 which compose the areas of the developing neural and 

 haemal arches are spherical and contain very large and 

 deeply stained nuclei; while those of the primary cen- 

 trum as well as those cells which comprise the hypo- 

 chordal bar are strikingly oval or spindle-shaped and 

 are more closely applied to each other. As yet the 

 scleroblastic cells do not appear in the area between the 

 notochord and the spinal cord, so that the primary cen- 

 trum is incomplete in that region. Since the primary 

 centrum in the alligator is formed entirely external to 

 the notochordal sheath, a considerable contrast exists 

 between that condition which maintains within the Elas- 

 mobranch fishes and certain other Chordates. In these 

 latter groups, openings occur in the notochordal sheath, 

 through which the scleroblastic cells may enter the 

 chorda and form the primary centrum within it. But 

 in the alligator, at this stage, these centra appear as a 

 series of membranous rings, incomplete dorsally, which 

 lie just external to the notochordal sheath and in the 

 same transverse plane as do the neural and haemal mem- 

 branous processes. These centra are separated from 

 each other by wide spaces, into which scleroblastic cells 

 will later migrate and which will subsequently be known 

 as the intercentra; so that the notochord presents at 

 this time a very characteristic moniliform appearance. 



These sclerotomes, together with the hypochordal bar, 

 constitute the beginning of a membranous vertebra. 

 (Fig. 6.) At first, in all the regions of the body, the 

 lower or haemal processes lie in approximately the same 

 transverse plane as do the neural processes; but in the 



