120 A. M. PATBRSON. 



into the limb bud, and below the terminations of the muscle- 

 plates. In successive sections, from within outwards, these 

 nerves can be traced to their terminations in the limbs. They 

 first spread out, and approach one another, as described ; in 

 doing so they unite with adjacent nerves, so that the next step 

 in the proceedings (fig. 9, b) is the formation of a plexiform 

 mass of nerve-tissue (plea?.), which encircles the artery. At 

 the same time that this plexus formation occurs the division 

 into dorsal and ventral branches is beginning. In the figure 

 the axillary artery in the centre, with a group of mesoblast 

 cells running up and down from it, shows the commencing 

 separation of the mass into these two portions. In sections 

 made a little farther outwards, the plexus gradually separates 

 completely (fig. 9, c) into a dorsal and a ventral mass (d. and 

 v.), each consisting of a broad, flattened band, separated by 

 the artery and some mesoblastic cells. Still farther out, just 

 before the limb is completely separated from the trunk 

 (fig. 9, d), the nerves appear as two distinct cords (d. and v.). 

 These gradually divide, become attenuated, and disappear as 

 they are traced towards the distal portion of the limb. Exactly 

 the same process occurs in relation to the nerves of the hind 

 limb. When oblique longitudinal sections are made at this 

 date, so as to cut through the length of the nerves (fig. 10), 

 they are seen to spread out and divide laterally, so as to unite 

 with similar branches (dorsal or ventral) of adjacent nerves, to 

 form the plexus at the root of the limb. As already stated, 

 there is no trace whatever at this date of either cartilage or 

 muscle in the limb, which consists entirely of undifferentiated 

 blastema. 



In embryos, four days twelve hours old, the nerves have 

 reached a more advanced stage of development. In the trunk 

 between the limbs (fig. 11, N.) the nerve, which twelve hours 

 earlier divided into a sheaf of branches in the body wall, now 

 splits into two well-defined and unequal branches at a point 

 just beyond the somato-splanchnopleuric angle. The larger 

 branch continues the direction of the main nerve, and can be 

 traced for some little distance between the muscle-plate and 



