386 
my’mer. so 
SI Bini Gui, GEM MO, Fig. 5. Frontal section in the ventral 
enoes EINE 5090 OY region of the myelon, showing the structure 
) 
09050 7809997029008 In 70%. vo = = . 
LESSONS SSID aL and relations (to somites) of the neuromerie ex- 
GED STH SOR RE ER pansions (myelomeres). Same stage as Figs. 3 
and 4. my’mer, myelomere; so., somite. 
Stes Wo 
00 
«0 N <5) SJ ee 
<0 \ 7, ORS o>. 4 = 2 
OO eee SS 
In those vertebrates in which the somites extend farther dorsad 
with reference to the neural tube (as in the chick) the constrictions 
of the myelon also have a greater dorsal extent. As soon as the 
somites lose their rounded form and no longer lie close to the neural 
tube, the constrictions disappear. If the myelomeres were ever 
structurally comparable with encephalomeres, the primitive resem- 
blances have been much modified. However, an important point 
favoring the view that the two are serially homologous is their 
uninterrupted sequence, a fact already insisted upon by Miss PLATT (89). 
To me the evidence of their serial homology with the encephalomeres 
depends not on their structure, but on their relation to somites and 
to motor nerves. To these relations I now turn. 
4, Somites of the Cephalic Region. 
The evidence of somites in the cephalic region of S. acanthias 
which I have to present is simply a confirmation of that already 
ee 
Fig. 6. A camera drawing of a cleared specimen of S. acanthias in the same 
stage of development as Figs. 3, 4 and 5, showing relations of encephalomeres, nerve 
