THE SOMITES OF THE CHICK 75 



edge of the lateral layer of the dermomyotonie of this somite, but 

 it seems to be converted into myoblasts like the upper and lower 

 edges of the lateral layers of the dermomyotomes of the trunk. 



The myotome has a clavicular form in vertical section, its upper 

 edge being slightly bent toward the neural tube and its lower edge 

 being strongly inclined laterally. The rapid expansion of the 

 mesenchyma arising from the cutis plate has apparently caused 

 the overlying ectoderm to bulge outward. The inner surface of 

 the myotome is quite sharp and regular and is only loosely at- 

 tached to the sclerotomic tissue. The outer surface, on the con- 

 trary, is closely connected with the overlying mesenchyma and 

 is somewhat irregular, owing to the formation of large irregular 

 spaces. These are most abundant somewhat below the center of 

 the plate, and appear to indicate the beginning of the separation 

 of the myoblasts to allow the interpolation of connective tissue 

 elements. Maurer's figure ('04, fig. 24) of the dorsal edge of one 

 of the dermomyotomes of the middle of the trunk of a chick em- 

 bryo of five days differs in certain respects from sections of this 

 and of all other dermomyotomes of the chick that I have seen. 

 The outer surface of the muscle plate is represented as regular and 

 well defined, not the inner surface; the medial or inner surface 

 is irregular, owing to the migration of sclerotomic tissue into the 

 muscle plate. Finally, the upper edge of the dermomyotonie is 

 very thin and acute, while in all the segments that I have seen 

 it is thick and rounded. I cannot explain the wide divergence 

 between Maurer's observations and my own. 



The lower edge of the muscle plate is not well defined but mer- 

 ges gradually with the adjacent connective tissue. It is slightly 

 indicated, however, by a blood-vessel which passes close under it 

 to supply the mesenchyma of the dermatome and which anasto- 

 moses with other vessels that pass between the slightly divergent 

 lower ends of the adjacent nwotomes. 



The nuclei of the lower part of the myotome, as before, are larger 

 and less closely packed than those of its upper part. The major- 

 ity of them are elongated longitudinally, but a few, particularly 

 those near the middle of the inner surface of the plate, are rounded 

 or are elongated in another direction. Similarly the axes of the 



