. Anatomischer Anzeiger. Anatomy, Comparative. 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. Ill—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 Balfoue's stage H five local thickenings of the lateral zones appear in the hindbrain region. Fi
. Anatomischer Anzeiger. Anatomy, Comparative. 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. Ill—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 Balfoue's stage H five local thickenings of the lateral zones appear in the hindbrain region. Fig. 4. Frontal section thorough the ventrajl zones of encephalomeres IV, V, VI. Same stage as Fig. 3. IV—VI, It is obvious that the encephalomeres I—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. Myelomer es. 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 Please note that these images are extracted from scanned p
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