188 



THE DEVELOPMENT OF THE CHICK 



Figs. 109 and 110, until complete union of the two takes place 

 (Fig. Ill) 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. 



C/zor Jy^. 



J 



Fig. 110. — Transverse section through the seventeenth somite of a 29 s 

 embryo. (Same embryo as Fig. 107.) 

 am. Cav., Amniotic cavity. E. E. B. C, Extra-embryonic body-cavity. 

 Gn., Ganghon. mes'n. V., Mesonephric vesicle. S.-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 



