CHAPTER VII.

GENERAL DEVELOPMENT OF THE TRUNK FROM STAGE K TO THE CLOSE OF EMBRYONIC LIFE, pp. 361-377.

_External epiblast._--Division into separate layers. Placoid scales. Formation of their enamel. _Lateral line._--Previous investigations. Distinctness of lateral line and lateral nerve. Lateral nerve a branch of vagus. Lateral line a thickening of epiblast. Its greater width behind. Its conversion into a canal by its cells assuming a tubular arrangement. The formation of its segmental apertures. Mucous canals of the head. Their nerve-supply. Reasons for dissenting from Semper's and Götte's view of lateral nerve. _Muscle-plates._--Their growth. Conversion of both layers into muscles. Division into dorso-lateral and ventro-lateral sections. Derivation of limb-muscles from muscle-plates. _Vertebral column and notochord._--Previous investigations. Formation of arches. Formation of cartilaginous sheath of notochord and membrana elastica externa. Differentiation of neural arches. Differentiation of hæmal arches. Segmentation of cartilaginous sheath of notochord. Vertebral and intervertebral regions. Notochord.