116 A. M. PATEESON. 



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 undif- 

 ferentiated 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 differentiation of the four dorsal 

 and ventral strata, which have been described as appearing 

 from the mesoblast-cells, at first undifferentiated, and forming 

 the original limb bud. In Chick embryos at six days, these 

 strata of ovoid cells have become more fusiform, and are col- 

 lected into more definite and separate systems. Two days 

 later the muscles throughout the body are seen distinctly. 



II. Development of the Spixal Nerves and Limb 

 Plexuses. 



The later development of the spinal nerves naturally divides 

 itself into two parts : firstly, in relation to the limbs ; and 

 secondly, in the trunk between the limbs. In essential points 



