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Fig. 3. A frontal section of an embryo of S. acanthias at a stage with 
28 post-otic somites, showing the local thickenings of the lateral zones of the neural 
tube in the region of the encephalomeres, ZZII—VII, encephalomeres; com. vag., vagus 
commissure; gn. acs-fac., Anlage of the acustico-facialis; gn. trg., Anlage of the 
trigeminus; *, posterior boundary of auditory invagination. 
section in the ventral region of encephalomeres IV, V and VI. Thus, 
at BALrour’s stage H five local thickenings of the lateral zones 
appear in the hindbrain region. 
En Se BSP 
Fig. 4. Frontal section thorough the ventrail ED GTR ALR TEER pags CMLL TO 
Ly i — 8 
zones of encephalomeres JV, V, VI, Same stage as ey) iF iene 
Fig. 3. IV—VI, encephalomeres. FO 
3 
It is obvious that the encephalomeres /—VII, as expansions of 
the neural tube involving ventral, lateral and dorsal zones, are not 
explicable as the passive results of mechanical flexure, or of pressure 
of mesodermal somites. The local thickenings in the region of 
encephalomeres III—VII can be accounted for only as the results 
of unequal local growth. My interpretation of them as important 
criteria of metamerism will be stated when I come to treat of their 
relations with nerves and somites. 
b. Myelomeres. 
In contrast with the expansions of the encephalon, the evidence 
presented by the expansions and constrictions of the myelon warrants 
the inference that the existence of the myelomeres is dependent 
upon the presence of the mesodermal somites, an explanation by no 
means possible for the encephalomeres. The myelomeres present no 
histological or structural conditions which warrant a different con- 
clusion. The constrictions of the myelon appear only after the somites 
are formed, and increase in number with the addition of new somites. 
They are situated opposite the somites (Fig. 5), and are confined to 
that portion of the neural tube against which the somites lie, i. e. the 
ventral portion. 
