ON THE FATE OF THE MUSCLE-PLATE. 109 



central mass the cells gradually become changed in character, being 

 more deeply stained in mass, and individually becoming oat-shaped 

 or fusiform. The long axes of the cells are directed from within 

 outwards. Two layers of cells thus appear, one above, the other 

 below the cartilaginous bar. The cartilage, however, is not yet 

 distinctly marked off, but is connected to these groups of cells by a 

 definite and still more deeply-stained (perichondrial) layer. The 

 nerves to the limbs derived from the plexuses have a very definite 

 relation to these central groups of cells, which are enclosed within the 

 nerve trunks. The dorsal nerves pass over, and the ventral trunks 

 beneath, this central area. Above and below the nerves two other 

 more distinct groups of ovoid cells (1 and 2) are now apparent, 

 collected each into more or less separate subsidiary bundles, and 

 easily distinguished from the surrounding undifferentiated mesoblast 

 by their shape, and by the fact that, being closely packed together, 

 they stain more deeply en -masse. The four simple, unsegmented 

 strata of ovoid cells — of which two are dorsal, and lie above and 

 below the dorsal branches of the nerves ; two ventral, and having a 

 similar relation to the ventral trunks — are evidently the precursors 

 of the muscular elements of the limbs. They are quite distinct from 

 the muscle-plates, and are separated from them by a large quantity 

 of undifferentiated mesoblast. The blood-vessels of the limb have 

 now attained a large size, and are regular in their arrangement. One 

 large artery (art.) passes down the centre of the limb on its ventral 

 aspect, lying among the nerves, and accompanied by a vein (V.). 



It is unnecessary to follow the muscle-plate further. It has been 

 shown that while it grows into the body wall between the limbs, and 

 forms the basis of the longitudinal muscles of the trunk ; in the 

 region of the limbs, it remains in its primitive position, and has no 

 share in the formation of the limb-muscles. It is merely concerned 

 here in forming the longitudinal muscles of the back. In the 

 limbs themselves the muscles are produced by the further differen- 

 tiation of the four dorsal and ventral strata, which have been 

 described as appearing from the mesoblast cells, at first undifferen- 

 tiated, and forming the original limb bud. In Chick embryos at six 

 days, these strata of ovoid cells have become more fusiform, and are 

 collected into more definite and separate systems. Two days later 

 the muscles throughout the body are seen distinctly. 



