OUTGROWTH OF THE NERVE FIBER 
799 
by the ordinary embryological methods.^ Cajal has iigured in a 
number of places his growth cones, as seen both in Golgi and in 
silver nitrate preparations. Those shown in his book on the Struc- 
ture of the Nervous S\stem, vol. 1, p. 515, are in most striking 
agreement with the figures here presented. Again, there is no 
sharp discrepancy between these figures and those of Held, 
whose figures, like those of Cajal are sharper than the present 
ones, since they represent the specific coloration of the neuro- 
fibrillae. The only essential difference shown by those of the 
former observer is in the relation of the young nerve fibers to the 
protoplasmic net- work between the cells and this to my mind is 
wholly a question of interpretation. Considering the uncertain 
nature of the intercellular net-work, as pointed out above, the 
unusually positive views of Held regarding its role in the develop- 
ment of the nerve fibers seem but ver}' insecurely founded. 
EXPERIMENTS UPON EMBRYONIC TISSUES ISOLATED IN 
CLOTTED LYMPH 
Description of methods 
The first attempt which I made to study the development of iso- 
lated bits of embryonic nervous tissue gave entirely negative 
results. The tissue was dissected out from the embryo and put 
either into physiological salt or Locke's solution, but no differen- 
tiation was observed, before disintegration began. Later a more 
natural environment for the isolated tissue was sought in the ven- 
tricles of the brain and in the pharynx of young embryos. The 
tissues were transplanted to these cavities and the specimens were 
killed after from two to seven days and examined in serial sec- 
* In salmon embryos preserved and stained by the ordinary embryological 
methods, no protoplasmic filaments are shown attached to the growing ends of the 
nerve fibers within the central nervous system, and for this reason the latter were 
figured as smooth in my paper on the histogenesis of nerves (Harrison '01). Ramon 
y Cajal has pointed out that this condition is likely due to the insufficiency of the 
methods. While I agree that there is some ground for this criticism, it seems never- 
theless probable that there are actual differences between the growing ends found 
in different places and in different species. 
