118 A. M. PATERSON. 



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 character 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 oat-shaped 

 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. 



3. On the homologies of the spinal nerves. 



Their development shows that the nerves which form the limb 

 plexuses are homologous with the whole nerves in the regions 

 between the limbs, where their arrangement is simplest, and not 

 merely with the lateral branch, as Goodsir supposed.* The nerves 

 in both regions first spread out into a ragged bundle. These bundles 

 at a later period arrange themselves into two well-defined cords, the 

 division from the main trunk having the same relative position in 

 both. In the regions between the limbs these trunks represent the 

 lateral and inferior branches ; in the regions of the limbs they are 

 dorsal and ventral in position. 



4. On the development of the limb plexuses. 



I have elsewhere shownf that in mammals, and as far as I have 

 been able to make out, the same holds good for birds also, the limb 

 plexuses are formed on a definite plan, which is essentially the same 

 in all the animals examined, and in relation to both fore and hind 

 limbs. The nerves which form the plexus divide, in the first place, 

 into dorsal and ventral branches. These divisions sub-divide, and 

 the secondary cords, whether dorsal or ventral, combine with the 

 cords formed by the division of adjacent (dorsal or ventral) trunks 

 to form the nerves of distribution. Any given nerve to the limb 



* " Edinburgh New Philosophical Journal," New Series, vol. v., Jan., 1857 ; 

 " Anatomical Memoirs," vol. ii., p. 201, 1868. 



t Graduation Thesis, Univ. Edin., 1886, " On the Spinal Nervous System in 

 Mammalia;" "The Limb Plexuses of Mammals," "Journal of Anatomy and 

 Physiology," vol. xxi., 1887, p. 611. 



