NERVE AND PLASMODESMA 69 
as heavy ‘fibrils’ as described by Paton. The affinity for neuro- 
fibrillar stains is especially marked at the lower, advancing 
extremity of the nerve anlage where Paton (’07) found the first 
indications of neurofibrillar substance in his preparations. From 
this evidence the conclusion seems warranted that the primary 
connections between neural tube and myotome are truly nervous, 
just as the experiments of Paton (’07) upon the reactions of 
elasmobranch embryos at the time the plasmodesmata are formed 
would lead us to infer. 
The answer to the third question is already implied in the fore- 
going discussion. As has already been stated, in stages before 
protoplasmic connection between tube and myotome is effected 
certain medullary cells in zones where later nerve anlagen make 
their appearance show in Bielschowsky-Paton preparations a 
deeply staining neuroreticulum. Such cells show a pronounced 
polarity and tend to assume a spindle shape. The staining 
material of the reticulum is peripheral in position and extends 
throughout the entire length of the cell, showing a tendency to 
become more densely aggregated at the outer extremity of the 
cell. Not infrequently the end of the cell appears to extend 
along the inner surface of the external limiting membrane of the 
tube just as if the neuroblastic cell were in the process of elonga- 
tion in the direction of its long axis and the movement of proto- 
plasm were deflected in direction by the resistance offered by 
the limiting membrane of the medullary wall (fig. 1). The 
neuroreticulum is densely aggregated in the elongated peripheral 
portion of some cells so as to appear in such as a heavy deeply 
stained neurofibril. 3 
In slightly later stages, when protoplasmic connection with 
. the myotome has been established in the manner described in 
detail by the writer in earlier papers (’14), similar neuroblastic 
cells with deeply staining processes are seen to extend into the 
nerve anlagen (figs. 3 to 5), that is to say into the so-called 
plasmodesmata. That these plasmodesmata are primarily 
formed by the protoplasmic outflow of similar neuroblastic 
cells has already been asserted by the writer (14. Note espe- 
cially figs. 4 to 7, pls. 1 and 2). The plasmodesmata, in other 
