MORPHOLOGY OF EYE MUSCLE NERVES 23 



The attempt has been made recently by Held ('06, '09) and 

 Paton ('07) to combine the two views of nerve histogenesis, 

 claiming a. primary connection of neural tube and myotomes by 

 means of protoplasmic bridges or plasmodesmata, along which, 

 as paths, later the true nerves — neurofibrillae — by centrifugal 

 (Held) or centripetal (Paton) growth — effect the definitive nerv- 

 ous connection with the muscle. True nervous connection with 

 muscle is thus secondary, but the connection takes place along 

 predestined paths — the plasmodesmata. 



WTiether the plasmodesmatous connections are secondary or 

 primary remains undetermined by Paton. Held (p. 277) holds 

 with the Hertwig brothers ('78) that they are secondary and he 

 states that Hensen's hypothesis that all nerves have arisen 

 through the incomplete separation of the origin and end cells 

 cannot be correct. Held, however, does not appear to have 

 established the truth of this assertion on the basis of a detailed 

 study of their genesis in the vertebrate embryo. The theoreti- 

 cal importance of the manner of establishment of these 'primary' 

 protoplasmic connections of neural tube and their genetic rela- 

 tions to the cells which form the neuro-fibrillae is so great that 

 it appears worth while to consider this portion of the histogenetic 

 problem with thoroughness and in great detail. The functional 

 importance of the primary protoplasmic connections has been 

 established by the valuable experiments of Paton ('07), who 

 demonstrated that in selachian embryos, in stages before the 

 appearance of neuro-fibrillae, the embryo responds by muscular 

 movements to external stimulus. If such reactions be effected 

 through the medium of the central nervous system, then it is 

 certain that the efferent paths of the impulses must be the 

 "undifferentiated protoplasmic bridges" described by Paton and 

 identified by him as the 'plasmodesms' of Held ('06), To some 

 this evidence might seem sufficient to prove their 'nervous' char- 

 acter, but not so to a disciple of von Apathy, to whom there 

 is ''no nerve without neuro-fibi411ae." With regard to these 

 protoplasmic bridges, Paton has the following to say (p. 555) : 



In the region of the ventral roots in a 5 mm. embryo of Pristiurus 

 there is a marked bulging forward of the protoplasm and in many 

 sections a 'bridge' al)out 4 to 5 /i in width similar to those shown by 



