104 A. M. PATERSON. 



previous embryologists have left them, to a point at which they may 

 be fairly compared with the adult state. 



Chick embryos, artificially incubated, have been used for the most 

 part. By means of series of continuous sections, cut in different 

 directions, the nerves have been traced from end to end in the different 

 stages of development, from their first appearance up to the forma- 

 tion of trunks having an arrangement closely similar to that found 

 in the adult. These sections have been compared with sections of 

 mammalian embryos of different ages, with the result that the condi- 

 tion of development of the nerves has been found to be identical in 

 both at periods in which the development of other parts and organs 

 of the body is the same. 



The methods adopted were in almost all cases the same. For 

 hardening the Mammalian embryos, Kleinenberg's solution of picric 

 acid was used ; for the Chick embryos, a cold saturated solution of 

 corrosive sublimate. A solution of borax carmine was the staining 

 agent employed, prepared according to Balfour's directions. The 

 sections were cut with a Jung's microtome, fitted with an ordinary 

 razor. 



The earliest stages in the development of the spinal nerves in the 

 Chick have been described by Marshall. He has shown that they 

 spring from the spinal cord as buds, of which the dorsal are the first 

 to appear, arising from the summit of the cord. The more anterior 

 of the dorsal roots arise from a "neural ridge," an elevation continued 

 back from the hind brain. By the interstitial growth of the dorsal 

 portion of the spinal cord these roots become separated, and their 

 attachments to the cord more laterally placed. The ventral roots 

 appear at a later date. Projecting outwards directly, they unite at 

 an acute angle with the dorsal roots to form the mixed nerve. The 

 spinal ganglion on the dorsal root is evident before the fusion of the 

 two roots occurs ; it is formed by the proliferation of the cells of the 

 bud which form the root. The spinal nerves are thus formed in 

 pairs, which occupy the intervals between the muscle-plates. 



Before tracing the further growth of the nerves from this point, 

 it is necessary to describe the destination of the muscle-plates and 

 the mode of formation of the limbs, as in their onward development 

 the nerves present differences according as they occur in relation to 

 the limbs, or in the intervals between them, 



