PAPERS OX BIOLOGY AND AGRICULTURE 215 



Immediately following the formation of the eight ver- 

 tebral elements there occurred a fusion of th^ two cranial 

 elements upon each side, as well as of the two caudal 

 elements. In each case cells of the cranineurals and 

 caudineurals grow ventrally to meet the cells of the 

 cranihaemals and caudihaemals ; so that as a result four 

 vertebral components may now be identified within each 

 segment. Partially following the terminology of Schau- 

 insland, the term cranial part-sclerotome is employed 

 to designate the fused cranineural and cranihaemal com- 

 ponent ; while the term caudal part-sclerotome likewise 

 identifies the fused caudineural and caudihaemal com- 

 ponent. Since the caudal vertebral component in each 

 case is considerably larger than the cranial, the term 

 part-sclerotome seems more adequate in its designation 

 than the term half -sclerotome of authors. 



Following the fusion of the original vertebral elements 

 to form part-sclerotomes, as above described, there oc- 

 curs a subsequent fusion of these part-sclerotomes to 

 form entire sclerotomes within which the definitive mem- 

 branous vertebra will later arise. The fusion of these 

 part-sclerotomes is alf ected as follows. The caudal part- 

 sclerotome, considerably larger than the cranial, extends 

 somewhat dorsalwards, and reaching more posteriorly 

 comes to unite with the smaller cranial part-sclerotome 

 of the next posterior segment. Thus it is evident that 

 entire sclerotomes are formed, not by the fusion of part- 

 sclerotomes within a segment but rather by a fusion 

 of part-sclerotomes of adjacent segments. So that 

 sclerotomes come to alternate in their position with the 

 myotomes from which the trunk musculature is to arise ; 

 affecting thereby the alternate relations of vertebrae and 

 muscles in the adult condition. Dorsally these sclero- 

 tomes extend to the level of the spinal ganglia, while 

 ventrally each reaches doT\iiward a very short distance 

 from the notochord. (Fig. 4.) 



In an embryo 7 mm. long, three parts of the above 

 described sclerotome may be identified. The upper more 

 narrow portion, which lies adjacent to the spinal cord, 

 is clearly the first stage in the development of the mem- 

 branous neural arch. Likewise, the lower or haemal por- 



