]18 A. M. PATEESON. 



body wall. In the region of the limbs (N., 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 arrange- 

 ment 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 separation 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 (iV., 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 (N., 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 

 satisfactorily the existence of the trunk passing to join the 

 sympathetic. A slender cord arises from the spinal nerve mid- 

 way 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 



