Fig. 7. Junction of a nerve and mucous canal. Stage P. Zeiss D, ocul. 2.
Fig. 8. Longitudinal and horizontal section through the muscle-plates and adjoining structures at a stage intermediate between L and M. The section is intended to shew the gradual conversion of the cells of the somatic layer of muscle-plates into muscles.
Fig. 9. Longitudinal section through the notochord and adjoining parts to shew the first appearance of the cartilaginous notochordal sheath which forms the vertebral centra. Stage N.
Fig. 10. Transverse section through the tail of an embryo of stage P to shew the coexistence of the rib-process and hæmal arches in the first few sections behind the point where the latter appear. Zeiss C, ocul. 1.
Fig. 11. Transverse section through the centre of a caudal vertebra of an embryo somewhat older than Q. It shews (1) the similarity between the arch-tissue and the hyaline tissue of the outer layer of the vertebral centrum, and (2) the separation of the two by the membrana elastica externa[246] (mel). It shews also the differentiation of three layers in the vertebral centrum: vide p. [374].
[228] Jenaische Zeitschrift, Vol. VIII.
[229] Entwicklungsgeschichte d. Unke.
[230] Urogenitalsystem d. Selachier. Semper's Arbeiten, Bd. II.
[231] Loc. cit. p. 398.
[232] Götte extends his statements about the lateral nerve to the nerves supplying the mucous canals in the head; but my observations appear to me, as far as Elasmobranchii are concerned, nearly conclusive against such a derivation of the nerves in the head.