THE ORGANS OF THE OUTER GERM-LAYER. 453 



structure of embryonic nerves is a point on which I should have 

 anticipated that a difference of opinion was impossible, had it not 

 been for the fact that His and KOlliker, following Eemak and 

 other older embryologists, absolutely deny the fact. I feel quite 

 «ure that no one studying the development of the nerves in Elasmo- 

 branchii with well-preserved specimens could for a moment be doubtful 

 on this point." Of the more recent investigators van Wijhe, Dohrn, 

 and Beard side with Balfour. 



Hensen has taken an entirely different view on the question of 

 the origin of the peripheral nervous system, one which differs from 

 that of KuPFFER, His, and KBlliker, as well as from that of 

 Balfour. He opposes the doctrine of the outgrowth of nerve-fibres 

 chiefly from physiological considerations. He can think of no 

 motive which is capable of conducting the nerves that grow out 

 from the spinal cord to their proper terminations — which shall 

 •cause, for example, the ventral roots always to go to muscles, the 

 ^dorsal roots to organs that are not muscular, and shall prevent 

 -confusion taking place between the nerves of the iris and those of the 

 eye-muscles, between the branches of the trigeminus and the acusticus 

 ■or facialis, etc. Therefore Hensen maintains on theoretical grounds 

 that it is necessary to assume that " the nerves never grow out to their 

 terminations, hut are always in connection with them." According 

 to his view, which he endeavors to support by observations, the 

 •embryonic cells are for the most part united with one another by 

 jneans of fine connecting filaments. He maintains that when a 

 <!ell divides the connecting thread also splits, and in this manner 

 there arises " an endless network of fibres." Out of these the nerve- 

 itracts are developed, while other parts of the network degenerate. 



The reasons given by Hensen are certainly worthy of great 

 attention. With further reflection on the subject they are easily 

 added to. If the nerves grow out to their terminal apparatus, why 

 do they not take the most direct course to their destination, why 

 are they often compelled to pursue circuitous paths, and why do they 

 ■enter into the formation of complicated plexuses of the greatest 

 variety ? whence are the ganglionic cells that are found to be 

 ■developed in considerable numbers even in the peripheral nervous 

 system in different organs, especially in the sympathetic nerve? In 

 ■order to make progress in this difficult field the peripheral nervous 

 ■system of Invertebrates must be taken into account more than it is 

 •at present, and in the investigation of embryos not only series of 

 sections but also other histological methods (surface-preparations of 



