PLATE 1 
EXPLANATION OF FIGURES 
All of the figures of this plate were drawn with Abbé camera, one-twelfth homo- 
geneous oil-immersion objective and no. 6 compensation ocular of Zeiss. In 
reproduction the mangification has been reduced by one-half. The series illus- 
trates stages immediately preceding and following the appearance of the so-called 
plasmodesmatous connections between neural tube and myotomes in the trunk 
region of Squalus embryos. In reality such ‘plasmodesmata’ are the anlagen of 
somatic motor nerves as demonstrated by their genesis, neurofibrillar structure 
(when adequately stained) and later histogenesis. As is well known to all stu- 
dents of elasmobranch embryos, there is no difficulty whatever in the determina- 
tion of stages as earlier and later, since such successive stages appear in succes- 
sive metameres of each embryo. In all metameres the somatic nerve anlagen 
make their appearance opposite the middle of the myotome. The embryos from 
which the drawings were made were prepared by the Bielschowsky-Paton method 
to demonstrate neurofibrillar structures. 
1 and 2 Portions of cross-sections of a 7-mm. Squalus embryo cut in the 
middle of metameres lying near the cloacal region. In the metameres immedi- 
ately anterior to the ones shown protoplasmic connection between tube and myo- 
tome has already been established by the migration of medullary protoplasm. 
In more posterior segments there is no protoplasmic connection whatever. The 
sections demonstrate the presence in the medullary wall of neuroblastic cells 
(cl.n’bl) containing a deeply staining neuroreticulum extending throughout the 
entire length of the cell. Such cells, as stated by the writer in an earlier paper 
(14), are the first to effect neuromuscular connections by means of protoplasmic 
outflow. Their nervous (neuroblastic) character is attested by their polarity 
and neuroreticular structure even before their neuraxone process extends outside 
the limiting membrane of the neural tube. They contrast in form, color, and 
structure with the adjacent cells of the medullary wall. In successful Bielschow- 
sky-Paton preparations the neuroreticulum is stained a deep blue-black, while 
the cytoplasm of adjacent cells is reddish-yellow in color. 
3, 4 and 5 Portions of cross-sections of Squalus embryos treated by the 
Bielschowsky-Paton method in metameres in which neuromuscular connections 
(‘plasmodesmata’) are already established. The nervous nature of the so-called 
plasmodesmata is evidenced by the presence of neurofibrillar material within 
them and the extension of neuraxone processes of neuroblastie cells into the 
‘plasmodesmata.’ Such evidence shows that instead of being undifferentiated, 
non-nervous bridges of protoplasm, into which or along which neurofibrils or 
neuraxones grow in order to effect neuromuscular connection, the ‘plasmodes- 
mata’ in reality are ab initio nervous structures. Non-nervous material is 
secondarily added to the nerve anlage by migration or outflow from the neural 
tube. 
ABBREVIATIONS 
ax,, neuraxone process new’ret., neuroreticulum 
cd., chorda dorsalis _ pl’sm., plasma 
cl.n’bl., neuroblast cell scl., sclerotome 
ms’ench., mesenchyma so., somite 
mb.cl., cell membrane tb.n., neural tube 
mb.lim., limiting membrane vac., vacuole 
my., myotome 
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