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, which therefore includes two 

 layers: the external cutis-plate or dermatome, and the internal 

 muscle-plate or myotome. With the elevation of the axis of 

 the body, the dermo-myotome gradually assumes a nearly vertical 

 position. 



Fig. no. — 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 



