388 
bution, but also the position of their nuclei (Kerne) within the 
wall of the neural tube. The relations of the motor nerves in the 
trunk region may be very briefly treated, since in this region the 
arrangements of motor roots and their nuclei within the neural tube 
is clearly metameric. They have their exit from the neural tube 
opposite the somites, i. e. in the region of constrictions between 
myelomeres. In Amphioxus and Petromyzon only, do I find the 
dorsal ganglia intersomitic in position. In the higher Vertebrates 
which I have studied the dorsal ganglia lie within and opposite the 
somites, and thus in the Vertebrate series assume a secondary relation 
to the mesoderm. 
In the head, where the nerve relations are less clear as regards 
their metamerism, it is necessary to study more carefally the 
development of the nerves and thus, if possible, to determine their 
primitive relations to the encephalomeres. I have already cited the 
evidence of the exact correspondence of the somites, as given by 
VAN WIJHE, with the primary brain vesicles (encephalomeres ). 
Disregarding for the present the first two encephalomeres (forebrain 
and midbrain), related as they are to those two mesodermal cavities 
which are most doubtfully somitic in character, viz. the anterior and 
pre-mandibular cavities, I will state briefly the evidence based on 
nerve relations that leads me to conclude that the hindbrain neuro- 
meres (encephalomeres III to VII) are good criteria of the number 
of primitive segments in this region of the head. 
In the young stages of S. acanthias two facts, so far as I know 
new, present themselves. In the first place, each of these five en- 
cephalomeres is the seat of a local thickening in the lateral zones of 
the neural tube‘). 
Secondly, from four of the encephalomeres, viz. Ill, V, VI 
and VII, are proliferated the ganglionic cells of the four cranial 
nerves wich innervate the first four visceral arches, viz. the trigeminus, 
the facialis, the glossopharyngeus, and the Urvagus?) (Fig. 6). From 
1) The thickening in the region of encephalomere III is later in 
differentiation than those in the region of encephalomeres IV—VI and 
is confined to the posterior half of the encephalomere. This local 
thickening, as well as that in the region of encephalomere VII, which 
is even later in differentiation, never beomes so sharply marked off as 
do the local thickenings in the region of the other three hindbrain 
encephalomeres. See Fig. 3. 
2) Horrmann (’94) has spoken of the paired segmental outpocketings 
of the neural tube in the region of the last three encephalomeres. 
