PAPERS OX BIOLOGY AND AGRICULTURE 217 



sacral and the caudal regions, it would appear, however, 

 as though the haemal process had shifted toward the 

 anterior margin of the segment. As a result, in these 

 posterior regions, this ventral part of the sclerotome lies 

 just posterior to the segmental blood vessel and adja- 

 cent to the ventral'root of the spinal nerve. In this stage 

 these haemal processes are more elongate in the tail 

 where they extend ventrally to a position lateral to the 

 caudal artery and vein. In the region of the trunk and 

 thorax, however, they are greatly reduced and appear 

 only as mere iiidiments ; while in the neck, there is no 

 evidence whatsoever of any haemal process. 



In an embryo 10 mm. long, a further migration of the 

 constituent cells of the notochord has taken place, so 

 that a clearly defined ei^itheliomoii^h layer is produced. 

 As a result the vacuoles are relatively larger than be- 

 fore. The notochordal sheath is relatively thicker than 

 before, and under higher magnification its fibrous com- 

 position is more evident. 



A marked increase in the relative thickness of the pri- 

 mary centmm is apparent at this stage. Differentiation 

 of the cells has resulted in the formation of two distinct 

 layers, the inner one of which lies just external to the 

 notochordal sheath. This inner layer is composed of the 

 long spindle-shaped cells which characterized the earlier 

 primary centrum, and it extends entirely around the 

 chorda connecting the bases of the neural arch processes 

 beneath the spinal cord. The outer relatively thinner 

 layer is composed of larger spherical scleroblastic cells 

 similar to those of the earlier part-sclerotome ; and they 

 are evidently a product of these part-sclerotomes to- 

 gether with additional cells from the original myotomes. 



In addition to these primary centra, primary intercen- 

 tra may now be identified. Cells, which comprise the lat- 

 ter, have apparently arisen from the original sclero- 

 tomes, and have migrated anteriorly and posteriorly 

 along the notochord, forming a series of intercentra 

 which alternate with the successive primary centra. 

 These intercentra, which lie opposite the original so- 

 matic mvotomes, differ from the centra in the absence of 



