ON THE FATE OE THE MUSCLE-PLATE. 125 



and ventral strata. In birds and mammals the same end is 

 reached without these preliminary steps, and without the 

 intervention of the muscle-plates. The definite relations 

 which these simple muscular layers bear to the nerves of the 

 limbs, throw light on the evolution of the limb plexuses. Each 

 nerve, passing into a particular region of the limb bud, divides 

 into dorsal and ventral branches, to supply the dorsal and 

 ventral surfaces respectively of that particular portion. As 

 the mesoblast forming the limb-bud becomes more differen- 

 tiated so as to give rise to the muscular layers, the portions 

 opposite to, and originally derived from, the same somites as the 

 nerves become fused, forming simple muscular layers in the 

 first place. The nerves therefore fuse together; the dorsal 

 branches forming a dorsal band, and the ventral branches a 

 ventral band, which pass out, and are finally lost in these 

 simple muscular layers. 



2. On the growth and development of the spinal nerves. 



It is difficult to demonstrate clearly, but it is next to impos- 

 sible to deny, that the spinal nerves are developed from 

 epiblast throughout their whole length. From the numerous 

 sections which I have examined at different periods of growth, 

 I have traced the spinal nerves, not only the nerve-roots, but 

 also the trunks and the plexuses, as a centrifugal growth from 

 the spinal cord. The growth of the nerves is both interstitial 

 and terminal. Consisting at first merely of rounded cells, in 

 an active state of proliferation ; in older embryos these become 

 first ovoid and then fusiform, at the same time being less 

 deeply stained with borax carmine. These fusiform cells, by 

 the alteration of their protoplasm, become converted into 

 nerve-fibres. Moreover, while this interstitial growth goes on, 

 the trunk of the nerve is elongated by means of proliferation 

 of the cells at the periphery, which retain a primitive cha- 

 racter longer than those in the more proximal portion of the 

 trunk. For example, when the cells are fusiform in the nerve 

 near the cord, they are oatshaped at the distal end; when 

 they are fusiform at the distal end of the nerve, they are fibrous 

 in the proximal part of the trunk. 



