68 H. V. NEAL 
their appearance later in the manner described below. Proto- 
plasmic connection between tube and myotome in Squalus is 
secondary and not primary. 
The failure of Paton, Held, and the writer to discover the 
presence of neurofibrillae in the first protoplasmic connections 
between neural tube and myotome led all three to assert that 
neurofibrillas are secondarily differentiated within non-nervous, 
undifferentiated ‘plasmodesmata.’ The divergent views as to 
the details of this process have been summarized by the writer 
(14, pp. 35-41) and need not be repeated here. In that paper 
the writer asserted that “medullary cells by a process of out- 
growth, form the first protoplasmic connection between tube 
and myotome” and that later within these processes the neuro- 
fibrillae make their appearance. Bielschowsky-Paton prepara- 
tions also showed the presence of a neuroreticulum within bipolar 
neuroblasts lying in the somatic column of the spinal cord and 
in the zones opposite the middle of the somites where later the 
plasmodesmata make their appearance. Similar neuroblasts 
containing a deeply staining reticulum are shown in figures 1 
and 2 of this paper. But at the time the 1914 paper was pub- 
lished no neurofibrillar structures were seen in the plasmodes- 
matous connections at the time of their first appearance. Since 
that time, however, they have been seen in a number of series 
prepared by the Bielschowsky-Paton method. Figures 3 to 5 
of this paper show the presence of neurofibrillar substance within 
the ‘plasmodesmata’ of Paton and Held. The justification for 
calling the deeply staining material of the plasmodesmata 
‘neurofibrillar’ consists in the fact that it stains precisely like 
the neurofibrillar network present within the neuroblasts which 
by their outgrowth form the plasmodesmata and in the further 
fact that it is possible to trace the neurofibrillar network of the 
neuroblastic cells from the tube into the plasmodesmata as is 
seen in the figures mentioned. Its granular appearance within 
the plasmodesmata may be interpreted as the result of cutting 
a fibrillar network transversely. In those instances where the 
neuraxone processes of the medullary neuroblasts are cut length- 
wise, as in figures 3 and 4, the neurofibrillar substance appears 
