ON THE FATE OF THE MUSCLE-PLATE. 113 



ceeding (Fig. 9, b.) is the formation of a plexiform mass of nerve- 

 tissue [plex.), 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 body cavity. The smaller nerve represents the 

 lateral branch of the adult, and is directed downwards and outwards 

 through the muscle-plate, on the outer side of which it divides and 

 is finally lost. The cord to the sympathetic nerve can be followed 

 farther than before ; but I have been unable to trace its connections 

 with the roots and trunk of the spinal nerves. 



In the limbs the early changes occurring in the blastema, which 

 lead to the production of tho osseous and muscular systems, have 

 already been described. The nerves are of large size, and can be 



