ON THE FATE OF THE MUSOLE-PLATE. 127 



following steps occur. The primitive nerve, in the first place, 

 grows out beyond the lower end of the muscle-plate, and 

 reaches the root of the limb. It there, secondly, spreads out 

 into an irregular series of processes, which pass into the undif- 

 ferentiated tissue of the limb. Thirdly, these branches at a later 

 date arrange themselves in two trunks, one dorsal, the other 

 ventral, which extend still farther into the limb and enclose 

 between them a mass of blastema, from which the cartilaginous 

 basis of the limb is formed. Fourthly, the dorsal and ventral 

 trunks fuse with adjacent dorsal and ventral trunks to form 

 two broad flat bands, from which, still later, the individual 

 nerves as found in the adult are produced. 



The development of the muscular system of the limb, 

 occurring after the formation of the nerves, corresponds with it 

 exactly. The muscles appear first as simple double dorsal and 

 ventral layers, among which the nerves pass as dorsal and 

 ventral bands, formed by the fusion of adjacent dorsal or 

 ventral divisions of the nerves of origin. As these muscular 

 strata lose their simplicity and take on the complex arrange- 

 ment of the adult, the nerves at the same time become more 

 and more subdivided, until in the adult the primitive characters 

 of both are considerably masked. 



Still, in the adult mammal, it is evident that the more pre- 

 axial nerves in the series supply the more preaxial portions, 

 the postaxial nerves the postaxial portions of the limb, 1 and 

 the combinations of dorsal divisions and ventral divisions of 

 the nerves are distributed to those muscular and cutaneous 

 areas which are derived respectively from the primitive dorsal 

 and ventral surfaces of the embryonic limb bud. 2 



In conclusion, I wish to express my deep indebtedness to 

 Professor Milnes Marshall, of Manchester, for much advice 

 and encouragement during the prosecution of the above re- 

 searches, and for his kind assistance in the preparation of the 

 present memoir. 



1 Herringham, "On the Human Brachial Plexus," 'Proc. Koy. Soc.,' Jan., 

 1887. 



2 ' Journal of Anatomy and Physiology,' vol. xxi, July, 1887. 



