122 A. M. PATERSON. 



as buds projecting downwards and outwards from the trunk, 

 and composed, for the most part, of undifferentiated blastema. 

 In the centre of this the cells are more closely massed together 

 than at the periphery, and are being arranged concentrically to 

 form the cartilaginous basis of the limb. Each of the roots of 

 the nerve divides into upper and lower branches, which respec- 

 tively unite ; the upper branches to form the superior primary 

 division, the lower to form the inferior primary division of the 

 nerve. The latter is the main trunk. Passing downwards and 

 outwards below the muscle-plates, it reaches the base of the 

 limb where it divides into two branches, one dorsal the other 

 ventral, with regard to the cartilaginous core. These branches 

 can be followed through the limb, actually as far as the epi- 

 blastic 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 on- 

 wards 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 charac- 

 teristic. 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 connec- 

 tive-tissue corpuscles lie among the fibres. 



At five days twelve hours the process of muscle and cartilage 

 formation in the limbs is more advanced. The appearance of 



