MULTIPLE NEUROMATA OF THE CENTRAL NERVOUS SYSTEM. 709 
form the neuro-fibrils and drive them forward, (b) conducting cells (Leitzellen), in 
the interior of which the neuro-fibrils pass. (2) The neuro-fibrils, which arise in 
the fibrillogenous zone of the neuroblasts, are continued into the interior of a system 
of pre-existing protoplasmic bridges, represented in the central nervous system by 
the network of the spongioblasts and in the mesoderm by the anastomosing ex- 
pansions (plasmodesmata) of star-shaped conducting cells (Leztzellen). (3) These Leit- 
_ zellen, which may possibly be of ectodermic origin and have the function of nourishing 
and protecting the axons, would become ultimately the cells of the sheath of Schwann : 
yet they are not capable of producing the neuro-fibrils. (4) In the earliest stage 
a nerve is non-nucleated; the primitive neuro-fibrils are enveloped in a granular 
neuroplasm which forms a broad and entirely non-nucleated zone. (5) The process 
of neuro-fibrillation is an intraplasmatic progression from the neurogenetic centre. 
(6) There scarcely exists any neurone independence, for the neuro-fibrils of one 
neuroblast penetrate into the interior of other neuroblasts, producing a diffuse 
network. 
The Hensen-Hetp hypothesis, therefore, is opposed to the unicellular genesis of the 
nerve fibre and the neurone teaching of the genetic unity of the ganglion cell and its 
ramifications, but it agrees with His in looking on the neuroblasts as the chief partici- 
pators in the formation of the nerve-path, z.e. from them proceeds the genetic impulse 
for the formation of nerve tissue. If we have read Hetp aright, he does not seem to 
have decided whether the neuro-fibrils formed in the initial nerve-path by the neuroblast 
go on being formed progressively by their influence on the plasma of the intercellular 
bridges, or whether the neuro-fibrils grow out within the plasma of the intercellular 
bridges by the driving forward action of the neuroblast. 
Casa (1907).—The classical illustrations in all modern text-books of the develop- 
ment of the embryonic nerve fibre are taken from Cagat’s works, and CaJat’s views are 
too well known to need any detailed statement. His has had no more loyal and con- 
vineing supporter than CasaL, whose beautiful silver preparations have conclusively 
proved to so many that the developing nerve fibre is the result of the continuous out- 
growth of the principal prolongation of the neuroblast of His. Caysau has shown that 
this prolongation has a free thickened end (céne de croissance) which glides between the 
cell interstices. This intercellular progression, in contrast to Hxxp’s intraplasmatic 
progression, takes place both in the interior of the embryonic nerve tube and in the 
depths of the mesoderm. The primary axon and the terminal cone have a neuro-fibrillar 
structure with an unstained neuroplasm and a fine limiting membrane. He has further 
shown that the cénes de croissance are entirely naked in their passage across the peri- 
medullary space. The first axons emigrating into the mesoderm are isolated, the latter 
are in intimate relation to one another. The adventitial cells (etzellen of Hexp, 
lemnoblasts of LenHossEK) are always between the bundles. To explain why the nerve 
fibres traverse the mesoderm and also their relation to the myotome and epithelium, 
Casau finds it necessary to suppose the existence of specific chemiotactic substances, 
