214 ILLINOIS STATE ACADEMY OF SCIENCE 



cranineural ; while the more ventral one, because of its 

 association with the blood vessels, wonld be known as a 

 cranihaemal element. On the same basis, then, the more 

 posterior of these elements in each segment would be 

 designated as the caudineural or the caudihaemal de- 

 pending upon its dorsal or its ventral position. These 

 terms were suggested by Professor J. S. Kingsley, with 

 whom the writer frequently conferred during the inves- 

 tigation. Thus it is evident that each body segment 

 would be characterized by a pair of cranineurals, a pair 

 of cranihaemals, a pair of caudineurals and a pair of 

 caudihaemals. This method of identification is entirely 

 satisfactory, because it not only affords an accurate de- 

 termination of the position of each element, but it also 

 does away with the use of such terms as pleurocentrum, 

 hypocentrum and areualia, the homologies of which are 

 so uncertain. 



In addition to the segmental blood vessels which mark 

 the limits of body segments, each cranineural and caudi- 

 neural bears a very definite relation to the nerve roots 

 of the spinal nerve. Passing backward through a seg- 

 ment, the order of sequence in the position of these parts 

 •is as follows. Just posterior to the segmental blood 

 vessel is the ventral nerve root ; this is followed by the 

 cranineural, the ganglion of the dorsal root, the caudi- 

 neural, and the next succeeding segmental blood vessel, 

 each in the order indicated. The cranihaemal and the 

 caudihaemal of each segment lie along the ventro-lateral 

 margin of the notochord, but in approximately the same 

 plane as do the corresponding neural elements. (Fig. 3.) 



Schauinsland (1906) in his description of the develop- 

 ment of the vertebral column of Sphenodon, describes 

 the single sclerotome or mass of scleroblastic cells upon 

 each side of a body segment. Subsequently, by means of 

 sagittal and frontal sections, he was able to demonstrate 

 a transverse cleft in each sclerotome; so that the terms 

 cranial half-sclerotome and caudal half-sclerotome were 

 employed to designate the resulting parts. Unfortu- 

 nately, in this study, frontal and sagittal sections were 

 not available, and such transverse clefts as Schauinsland 

 describes were not identified in my material. 



