114 A. M. PATERSON. 



traced more than half-way through the limb towards the distal end. 

 At the root of the limb the main trunk (Fig. 12, N.) can be seen in 

 transverse sections to divide into two well-defined trunks, which are 

 clearly homologous with the terminal branches of the nerve in the 

 region of the trunk at this date (Fig. 11). These two large nerves 

 are respectively dorsal (d.) and ventral (v.) ; and enclose between 

 them the densest portion of the blastema (3). This enclosed portion 

 has already been described as consisting of three parts — a central 

 part, which is going to form cartilage, and a dorsal and ventral part, 

 the elements of muscular tissue. On the dorsal surface of the dorsal 

 nerve, and on the ventral surface of the ventral nerve, are other 

 layers of mesoblast cells (1 and 2) undergoing division preparatory 

 to the production of muscles. In longitudinal sections the three 

 main nerves supplying the fore limb can be traced as before in 

 successive sections, each dividing into dorsal and ventral branches ; 

 and these again uniting with adjacent nerves to form two flat- 

 tened bands, which pass to the dorsal and ventral surfaces of the 

 limb. 



The nerve-trunks are almost entirely fibrous now, with rows 

 of deeply-stained nuclei arranged among the wavy fibres. These 

 are evidently the connective-tissue elements of the nerve-trunks. 

 Towards their terminations the fibres are fewer and the fusiform 

 nerve-cells more abundant. 



In Eat embryos between twelve and fourteen days old, exactly 

 the same condition of development of the nerves is found as in the 

 Chick at four days twelve hours. The state of development of the 

 body generally is the same, the limbs exist as buds projecting down- 

 wards 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 respectively 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 



