ON THE FATE OF THE MUSCLE-PLATE. Ill 



difference in the relative amount of the growth of the nerves. The 

 whole embryo has, however, increased in size. In the trunk (Fig. 5) 

 the nerves cannot yet be traced into the body wall. In the region 

 of the limbs (A T ., Fig. 6) they have passed out beyond the lower end 

 of the muscle-plate and beyond the angle of the body cavity. The 

 marked change, however, at this date, is in the histological structure 

 of the nerves. The spinal ganglia are well-formed ovoid masses, 

 composed of large ovoid cells with two nuclei, the cells having a 

 general arrangement in vertical rows. The cells forming the nerve- 

 trunks have become elongated, fusiform, with fibrillar processes at 

 each end. The body of the cell does not stain well ; the nucleus, 

 large, oval, and with several nucleoli, lies in the centre of the cell, 

 and stains deeply; the distal ends of the nerves in the regions of the 

 limbs present a ragged appearance, due to the protrusion and sepa- 

 ration of these spindle-shaped cells into the mesoblastic tissue. 



At four days the histological change is more marked. The cells 

 forming the nerve-trunks have become more fibrous, the nuclei are 

 less numerous, and the trunks stain yellow en masse. Both between 

 the limbs and in relation to the limb buds the growth of the nerves 

 has continued. In relation to the limbs (A 7 "., Fig. 8), the nerves 

 sweep round between the lower ends of the muscle-plates and the 

 body cavity, and, reaching the base of the limb, spread out, and then 

 divide into a sheaf of branches, which diverge in the formative 

 blastema of the limb. In the trunk between the limbs the nerves 

 (A 7 "., Fig. 7) have extended a great way down the body wall, lying 

 between the muscle-plate and the body cavity, but not reaching as 

 far as the lower end of the plate. They divide, as in the limb, 

 into branching processes, some of which pass directly outwards into 

 the muscle-plate and divide again ; some pass on, lying within the 

 muscle-plate. 



It is at this period that I have first been able to make out satis- 

 factorily the existence of the trunk passing to join the sympathetic. 

 A slender cord arises from the spinal nerve midway between the 

 junction of the roots and the distal end. It courses inwards at right 

 angles to the main trunk, and is soon lost. Now also the formation 

 of the superior primary division of the nerve is first seen distinctly. 

 It is constructed in the same way in mammals, and is seen still 

 more clearly in Iiat embryos at fifteen to seventeen days (Fig. 1G). 



