THE MESOBLASTIC SOMITES. 823 



strongly with the loosely arranged stellate cells of the ventral 

 part of the somite. 



The cells forming the deeper, or ventral, walls of the muscle- 

 plates become elongated parallel to the axis of the embryo, and 

 very soon become converted into a system of longitudinal muscle 

 fibres. In this way a broad band of longitudinal muscle fibres 

 is formed, stretching along the whole length of each side of the 

 body of the embryo, each band being divided transversely, by 

 the original protovertebral lines of segmentation, into paired 

 muscle segments or myotomes. These muscle bands in the 

 chick correspond to the great longitudinal body muscles of 

 Amphioxus or of Fishes ; and from them a great part of the 

 voluntary muscular system is developed. 



The muscle-plates at first lie nearly horizontally, their inner 

 borders being at a level slightly dorsal to their outer edges. 

 As the body of the embryo increases in thickness, and assumes 

 more definite shape, the muscle-plates become placed more and 

 more obliquely (cf. Fig. 129) ; and by the end of the third 

 day (Fig. 124, mp) they lie almost vertically, the original inner 

 border becoming dorsal, and the original outer border ventral 

 in position. 



The outer, or ventral, edge of each muscle-plate rapidly ex- 

 tends into the somatopleure, and by the fifth day has spread half 

 way down to the ventral surface of the embryo ; the greater part 

 of its component cells become converted into fusiform muscle- 

 cells, from which the muscles of the back and trunk are deve- 

 loped. At the dorsal and ventral borders of the muscle-plates 

 the cells retain their epithelial character so long as the plate 

 continues to grow. 



In the segments opposite the limb buds (Fig. 115), the 

 muscle-plates stop at the bases of the limbs ; the muscles of the 

 limbs themselves are formed independently of the muscle- 

 plates, about the fifth or sixth day, an arrangement which is 

 probably to be regarded as a modification of a more primitive 

 one by which the musculature of the limbs is derived directly 

 from that of the body. 



