The Reactions of the Vertebrate Embryo to Stimulation etc. 545 



Prior to the period about to be described neìther the peripheral 

 uor centrai nerrous system may be said to contain any fuUy dif- 

 ferentiated uerve tracts. and yet. as will be shown later on. paths 

 do exist by whìch impulses may be received and transmitted. 



Wheu the embryos have reached a length of from 7 — 8 mm. 

 the development and differeutiatìon of the various elements in 

 the centrai as well as peripheral nervous system proceed with 

 remarkable rapidi tv, and I know of no phenomena more striking 

 than the siidden change that occurs in the appearance of the nervous 

 system in Selachìan embryos between the length of 5 and 8 mm. 

 In the former stage, as will be shown later on, the merest rudiments 

 of a nervous system exist, so primitive and detached that from the 

 morphological standpoint one is at tìrst inclined to regard tbem as 

 negligible elements. Theu a raarvellous change begins and almost 

 instantaneously in the brain , spirai cord , and ganglia as well as 

 in the peripbery, conntless neurofibrils running either in nerve 

 bundles or each one quite free, seem suddenly to be called into 

 existence. 



In a well stained series of sections through a Torpedo embryo 

 of 7 mm. in length the following conditions may be noted. 



No neurotìbrils are seen in tbe first few sections of this 

 series extending through the mid-brain , then as one passes a 

 few coarse comma -like tìlaments begin to make their appearance 

 in the peripbery and. by the time a poiut in the series has been 

 reached correspouding to the piane just above the eyes, a single 

 bündle of fibrils may easily be made out extending from the dorsal 

 almost to the ventral end of that part of the mid-brain contained 

 in the section. In each piane, it may easily be seen that these 

 primitive bundles are frequently made up of two or three fibrils 

 which seem to spring in the majority, if not all instances, from cells 

 lying tangentially and the only ones to be found in this zone. A 

 namber of black dots representing undoubtedly the cross sections of 

 fibrils running longitudinally are noticed in the jieriphery. The 

 fibrils conneeting tbe cranial with the caudally situated portions of 

 the brain become more numerous as one approaches the point of 

 origin of the fifth nerve while the bundles lying along in the mar- 

 ginai veil zone (Randschleier) diminish rapidly in nnmber. In 

 sagittal sections a few fibrils may be noted marking the beginning 

 of the Oculomotorius. The process of neurofibrillation in tbe Trige- 

 minus has already progressed considerably. A few fibrils eoming 



35* 



