82 LEONARD W. WILLIAMS 



notochordal processes of the sclerotome into a nearly horizontal 

 position. The aortic process has been carried inward and is now 

 a nearly vertical septum between the aorta and the posterior 

 cardinal vein. 



In embryos of thirty segments, the derniomyotome of the tenth 

 somite is quite separated from the sclerotome and is inclined 

 downward and laterally approximately at an angle of 45° with the 

 sagittal plane of the embryo. Its breadth (255 microns) is nearly 

 twice its length (150 microns). The ventro-medial surface of the 

 derniomyotome is strongly convex except at the upper and lower 

 edges where it is barely concave. Its dorso-lateral surface is 

 slightly convex, both at the edge and at the center, and the inter- 

 mediate zone is flat or barely concave. The central convexity 

 of both surfaces of the dermomyotome is due apparently to the 

 expansion of the tissue of the dermatome as it becomes mesen- 

 chyma. A small central area of the dermatome is entirely con- 

 verted into mesenchyma and a circular zone as broad as the 

 dermomyotome is long and somewhat more than half its breadth 

 is partly so. The lower edge of the dermomyotome rests upon 

 the posterior cardinal vein and is partly separated from the 

 ectoderm by a thin sheet of compact mesenchyma belonging to 

 the parietal plate but its slight contact with the ectoderm forms a 

 definite boundary which proves beyond doubt that the mesen- 

 chyma in the concavity of the muscle plate cannot have migrated 

 from the parietal plate. The increased breadth of the dermo- 

 myotomes seems to be due to two factors— intrinsic growth and 

 the addition of new material at its dorsal growing edge. The 

 cavity of the segment is represented by small cavities in the upper 

 and lower edges of the dermomyotome. 



The sclerotome is fusing with the adjacent sclerotomes and its 

 tissue is becoming vascularized. 



Figure 17 represents a frontal section through the tenth somite 

 of an embryo of forty segments. It is remarkably like figure 120, 

 p. 205,of a somite of the rabbit inMinot's "Human Embryology." 

 The dermomyotome, which in embryos of thirty segments was 

 inclined approximately at an angle of 45° with the median plane, 

 is now nearly vertical. The plane of the section is slightly inclined 



