66 H. V. NEAL 
stated by Bidder and Kupffer (57). Furthermore, among those 
who hold that neuromuscular connections are secondary there 
is disagreement as to whether such connections are effected by 
indifferent (glia or neurilemma) cells, as suggested by the Hert- 
wigs (’78) and recently maintained by Held (’09), or by the out- 
growth of the neuraxone processes of medullary neuroblasts, 
as believed by advocates of the Bidder-Kupffer hypothesis. 
Not until these questions are adequately answered is it likely 
that the investigation of neurogenesis will cease. 
The invention in recent years of specific neurofibrillar stains 
and the demonstration of neurofibrillae in the earlier stages of 
neurogenesis have led to the general adoption of the aphorism 
of Apathy that “there is no nerve without neurofibrillae”’ as a 
criterion of nervous structure. How greatly needed such a 
criterion has been is only too well known to those familiar with 
the literature of neurogenesis. In the absence of such a criterion 
of nervous structure, it has been hitherto possible to find in any 
cellular strand extending to or from the nervous system the 
evidence of ‘primitive nerves’ wherever they were demanded for 
schemes of ancestral metamerism. However, notwithstanding 
the application of improved methods of demonstrating neuro- 
fibrillae in embryonic nervous tissue, it has been found impossible 
up to the present to demonstrate the presence of neurofibrillae 
in the primary connections between nerve and muscle, that is 
to say, between neural tube and myotome. The existence of 
such neurofibrillae in the plasmodesmata of Squalus embryos 
is asserted in this paper for the first time in vertebrate embryos.! 
The present paper raises three controverted problems in nerve 
histogenesis: 
1. Are the connections between nerve and muscle primary 
or secondary? 
2. Are neuromuscular connections primarily undifferentiated 
plasmodesmata, as asserted by Paton (’07) and Held (’09), or 
are they primarily neurofibrillar? 
1A preliminary report of the conclusions reached in this paper was made 
before the American Society of Zoologists at Philadelphia, December 29, 1914, and 
an abstract reprinted in Science, 1915, vol. 41, pp. 485-486. War work has delayed 
the appearance of the final paper. 
