n NERVE DEVELOPMENT 103 



It is to be noted in passing that Kupffer's original observations 

 were made upon Mammals and those of His (1868) upon the Fowl. 

 In other words, in both cases the embryos were such, in regard both 

 to the minute size of their cell elements and to their high position 

 in the Vertebrate scale, as to be unsuited to provide a reliable basis 

 for the generalization that has been built upon them. 



The His view as expounded- by one of its most distinguished 

 supporters S. Eamon y Cajal (1909) may be summarized as follows, 

 the case of the motor nerves being taken for the sake of simplicity. 

 Each motor nerve-fibre arises as an outgrowth from a neuroblast, or 

 young ganglion -cell, lying within the spinal cord. The fibre sprouts 

 out from the neuroblast, makes its way to the surface of the spinal 

 cord, perforates that surface, and proceeds to grow freely through the 

 mesenchyme. The free end of the fibre forms a " cone of growth," 

 commonly shaped somewhat like a grain of barley and with a 

 pointed end. This "cone of growth" shows an active amoeboid 

 movement, by which it insinuates itself through the interstices of 

 the mesenchyme. Sometimes it may be seen to flatten or mould 

 itself against obstacles in its path. 



In the Fowl these processes take place during the third and 

 fourth days. Eventually (about the fifth day, in the Fowl, in 

 most cases) the growing nerve -fibres reach their destination and 

 become joined up to the muscle cells which form their definitive 

 end-organs. 



The essential feature of the His view is that the nerve-fibre 

 (which already shows the characteristic specific reactions of a nerve- 

 fibre, e.g. on impregnation with silver salts, and is therefore not 

 merely a strand of undifferentiated protoplasm) sprouts out from 

 the central nervous system and grows through the intervening 

 mesenchyme with a free end until it becomes joined up secondarily 

 with the end-organ. 



The His view at the present day rests upon a large body of 

 observed facts. In studying the embryology of almost any Verte- 

 brate it is easy to find in sections what appear to be freely ending 

 nerve-fibres sprouting out from the spinal cord. Some of the most 

 beautiful preparations of this kind have been made by Eamon y Cajal 

 and others by the use of silver impregnation methods. 



Perhaps the most striking evidence, which has recently been 

 adduced in favour of the His view, has been obtained by experi- 

 mental methods, especially by Harrison (1908, 1910). In one set 

 of experiments which have been regarded as particularly convincing 

 Harrison removed small pieces of embryonic spinal cord from Frog 

 embryos at a period just before that at which the motor nerves 

 became visible, and was able, by using ordinary antiseptic pre- 

 cautions, to keep them in a living condition for relatively prolonged 

 periods mounted in a drop of sterile lymph under a coverslip upon a 

 slide. The lymph soon clotted and held the piece of spinal cord in 

 position. Harrison now observed in many cases little projections 



