188 THE DEVELOPMENT OF THE CHICK 
Figs. 109 and 110, until complete union of the two takes place 
(Fig. 111) and there is established a complete dermo-myotomic 
plate in each somite. This is usually known as the myotome, 
which therefore includes two layers; the external cutis-plate 
or dermatome, and the internal muscle-plate. With the eleva- 
tion of the axis of the body, the myotome gradually assumes 
a nearly vertical position. 
Fic. 110. — Transverse section through the seventeenth somite of a 29s 
embryo. (Same embryo as Fig. 107.) 
am. Cav., Amniotic cavity. E. E. B. C., Extra-embryonic body-cavity. 
Gn., Ganglion. mes’n. V., Mesonephric vesicle. $8.-Am., Sero-amniotic con- 
nection. Other abbreviations as before. 
Other details concerning the early history of the sclerotome 
are given in Chapter XIII, and it remains to add here only a short 
description of certain changes in the cells of the myotome (myo- 
blasts). In longitudinal sections the cells of the myotome are 
seen to become spindle-shaped soon after the folding towards 
the dermatome begins. The nuclei of the myoblasts are large 
and stain less deeply than those of adjoining tissues. They 
become elliptical in correspondence with the form of the cell- 
bodies. Each myoblast soon stretches from anterior to pos- 
terior faces of the somite, and this represents the first stage in 
the differentiation of the voluntary muscles. 
In later stages the myotomes send outgrowths into the limb- 
buds and ventral body-wall for the formation of the voluntary 
