ON THE FATE OF THE MUSOLE-PLATE. 123 



the nerves in transverse section is much the same as before. 

 In successive longitudinal (sagittal) sections (fig. 15) the 

 nerves can be seen at the root of the fore limb, undergoing 

 division and union in the brachial plexus in the same way as, 

 but more definitely than in younger embryos (four days six 

 hours, fig. 9). The three main trunks are seen first (fig. 15, a, 

 N., 1, 2, 3) in company with the axillary artery and vein. They 

 then divide, in successive sections (figs. 15, b — 15, e), into 

 dorsal (d.) and ventral (v.) branches. The dorsal branches unite 

 with dorsal branches, the ventral branches with ventral branches, 

 to form the nerves of distribution to the limbs. The plexus- 

 formation is now complete, and from this time onwards there 

 is no change in the essentials of its formation, which tally with 

 the condition of the adult brachial plexus. It is to be borne 

 in mind that, though the plexus is completely formed, yet the 

 muscular elements are in a simple condition. Muscles are not 

 formed in anything approaching the complexity of the adult 

 limbs sooner than the ninth day. 



III. Conclusions. 



1. On the fate of the muscle-plate and development of the 

 muscles of the limbs. 



In Elasmobranchs the muscles of the limbs are formed by 

 the muscle-plates which grow down, and as they pass the roots 

 of the limbs give off small buds, which become separated off 

 and form the starting point of the muscle formation in the 

 limbs. 1 In higher Vertebrates, while it has been held probable 2 

 that the muscle-plates do not enter into the formation of these 

 muscles, it has not been shown satisfactorily how they do arise 

 and what becomes of the muscle-plates. It is evident from the 

 foregoing details that the muscle-plates, in the regions of the 

 limbs, stop short in their downward growth, do not pass farther 

 than the base of the limb, and are not concerned in any way 



1 Balfour, ' A Monograph on the Development of Elasmobranch Fishes,' 

 London, 1878. 



2 Kolliker, * Entwicklungsgeschiehte d. Menschen u. der hoheren Thiere,' 

 Leipzig, 1879. 



