II 



NERVE DEVELOPMENT 



119 



represented by the two ends of the motor trunk were originally in 

 close proximity, and a condensed strand of the network joining the 

 two points would naturally be left as a bridge when they became 

 separated by the deepening of the cleft between mesoderm and 

 endoderm (cf. Fig. '66). 



To sum up, in regard to the mode of development of the nerve- 

 trunks, it seems reasonable in the present state of our knowledge 



(1) to reject definitely that portion of the Hensen view which 

 looks on the protoplasmic bridges 



as having persisted from the com- 

 mencement of segmentation, 



(2) to regard the His view of 

 free-ending fibrillated outgrowth as 

 non-proven and for various reasons 

 improbable, 



(3) to believe that the nerve- 

 trunk already exists as a proto- 

 plasmic bridge between centre and 

 end-organ at a period when these 

 are still in immediate contact, even 

 although this has up to the present 

 been definitely shown by actual ob- 

 servation only in a few peculiarly 

 favourable instances, 



(4) to leave the exact period at 

 which the protoplasmic bridges come 

 into existence an absolutely open 

 question as being beyond the limit 

 of reliable observation, 



(5) as regards the sheath of 

 Schwann, to accept the view that it 

 is derived from mesenchyme. 



It will be noticed that little has 

 been said so far regarding the mode 

 of development of the actual neuro- 

 fibrillae. Their origin is indeed 

 unverifiable by direct observation, 

 with any certainty, owing to their 

 minute size. They appear to spread 

 outwards from the centre, and 

 Held interprets this appearance by a kind of His theory on a 

 minute scale, holding that each fibril grows out with a free end 

 through the protoplasmic bridge. On the other hand if it be the 

 case as suggested on p. 112 that the fibrils simply represent the 

 specialized paths of nerve impulses there would be nothing surprising 

 in their becoming visible first in the neighbourhood of the ganglion- 

 cell from which the impulses start and from which also is exercised 

 control over the metabolism of the nerve-trunk. Were this the 



Fig. 66. — Illustrating the structure of a 

 hypothetical primitive Vertebrate at 

 a time when the protostoma was still 

 open. In the lower figure an entero- 

 coelic pocket, the rudiment of a 

 mesoderm segment, is becoming de- 

 marcated from the rest of the endo- 

 derm by the downward spreading of 

 a split between the points a and b. 

 In the earlier stage shown in the 

 upper figure this split has not yet 

 begun to develop, and the points a 

 and b are seen in close proximity to 

 one another on the outer surface of 

 the endoderm. 



m.p, medullary plate ; p.s, protostoma. 



