210 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
b. Nerve RELATIONS IN THE TRUNK OF S. ACANTHIAS, 
An examination of sections in the trunk region of embryos of S. acan- 
thias leaves no doubt whatever that the chief proliferation of ganglionic 
cells occurs in the regions of constriction between myelomeres, i. e. 
opposite the somites, and that the ventral roots also arise opposite the 
somites. Motor roots appear long before the sensor roots, as was first 
stated by Sagemehl (’82). Dohrn has affirmed that they arise as early 
as Balfour’s stage H. I find them in embryos of S. acanthias in which 
34 somites are differentiated, stage H. From the very first, i.e. at 
this early stage, they are in relation with the ventral portion of the 
neural tube at a point directly opposite the middle of the somite. That 
the relation with the tube is opposite the middle of the somite is most 
easily demonstrated in frontal sections (see Plate 6, Fig. 42, which 
represents a frontal section of an embryo with 50 somites); but that 
their relation is with the ventral wall of the tube, is most clearly seen 
in cross sections (Plate 6, Fig. 41, rx. v.). In frontal sections more 
dorsally situated than those which show the ventral roots, the spinal 
ganglia are likewise seen to lie opposite the middle of the somites! 
(Plate 6, Fig. 43). ‘In later stages, however, the spinal ganglia lie 
opposite the anterior portion of the somites, i. e. intersomitic in position, 
as a result, probably, of the shifting of the somites. Since by this time 
the constrictions between myelomeres have disappeared, út is quite im- 
possible to state that dorsal roots arise either from the constrictions or from 
the dilatations of the myelomeres. 
McClure (’90, p. 42) has said that in the forms studied by him “the 
dorsal branches of the spinal nerves pass from the external surface of the 
myelomeres to the space between two somites, which is opposite their 
point of origin, and fuse with the epiblastic thickenings to form the 
spinal ganglia.’ Such a statement, if true, is certainly of great impor- 
tance in settling the question of the morphology of cranial nerves. For 
it is now generally stated by morphologists that the chief distinction be- 
tween spinal and cranial nerves consists in the fact that the ganglia of 
cranial nerves receive cellular material during development from the 
ectoderm of the lateral surface of the head, whereas the spinal ganglia 
do not. So far as I know, McClure’s statement remains unconfirmed, 
1 Similar relations of dorsal ganglia and ventral roots have been shown by Mar- 
shall (’78, Plate III. Figs. 27 and 28) for birds; by Hoffmann (’90, Taf. CLV. Fig. 7) 
for reptiles; by Dohrn (’91, Taf. V. Figg. 16 und 17) for Selachii; and by Sewert- 
zoff (95, Taf. V. Fig. 16) for Amphibia. 
