ON THE FATE OF THE MUSCLE-PLATE. 115 



followed through the limb, actually as far as the epiblastic surfaces 

 and almost to the distal end. In minute structure the trunks very 

 much resemble the nerves in the Chick, the chief difference being 

 that the fusiform cell elements are more evident throughout. 



In Chick embryos at five days the changes in the nerves between 

 the limbs are not marked (Fig. 13, N.). The lateral and inferior 

 branches are well defined, and the whole nerve has passed farther 

 down the body wall. From this time onwards these trunk-nerves 

 present no marked differences in morphological arrangement from 

 what is found in the adult. 



In the regions of the limbs at this date, as already described, the 

 cartilaginous basis and muscular elements have begun to make their 

 appearance. The nerves themselves occupy a position with regard 

 to these elements which is highly characteristic. The dorsal and 

 ventral trunks (Fig. 14, d. and v.) are each covered above and below 

 by masses of specialised, oat-shaped cells, which represent the layers 

 of dorsal and ventral muscles. These double dorsal and ventral 

 muscular layers are also separated by the cartilaginous framework of 

 the limb. The nerves themselves stain yellow; consist of extremely 

 wavy fibres, and present no distinct nuclei. Deeply-stained con- 

 nective-tissue corpuscles lie among the fibres. 



At five days twelve hours the process of muscle and cartilage for- 

 mation in the limbs is more advanced. The appearance of 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 



