186 THE DEVELOPMENT OF THE CHICK 
(Fig. 107) the primitive relations of the parts are still preserved. 
In the twenty-sixth somite (Fig. 108) it will be seen that the 
cells of the core and of the ventral and median wall of the somite 
extending from the nephrotome to about the center of the neural 
tube are becoming mesenchymal; they spread out towards the 
notochord and the space between the latter and the dorsal aorta. 
These cells constitute the sclerotome. The muscle plate extends 
from the dorsal edge of the sclerotome to the dorso-median angle 
of the wall of the somite, and the dermatome from this point 
to the nephrotome. 
Fic. 108. — Transverse section through the twenty-sixth somite of a 29s 
embryo. (Same embryo as Fig. 107.) 
Derm., Dermatome. My., Myotome. Scler., Sclerotome. V. ¢. p., Pos- 
terior cardinal vein. Other abbreviations as before. 
Fig. 109 is a section through the twentieth somite of the same 
embryo. The sclerotome is entirely mesenchymal, and its cells 
are extending between the notochord and aorta, and along the 
sides of the neural tube. The muscle-plate has now bent over 
so that its inner surface is being applied against the dermatome, 
but there is still a considerable cavity (myoccele) between the 
two, at the lateral angle of the dermo-myotomic plate. The 
lateral edge of the dermatome is freed from the nephrotome, and 
turns in to a slight extent. Other details are readily understood 
from the figure. 
The growth of the free edge of the muscle-plate towards the 
free lateral edge of the dermatome continues as illustrated in 
