CHAPTER VI.. Development of the Trunk During Stages G to K, Pp. 315-360.

DEVELOPMENT OF THE TRUNK DURING STAGES G TO K, pp. 315-360.

Order of treatment. External epiblast.--Characters of epiblast. Its late division into horny and epidermic layers. Comparison of with Amphibian epiblast. The unpaired fins. The paired fins.--Their formation as lateral ridges of epiblast. Hypothesis that the limbs are remnants of continuous lateral fins. Mesoblast.--Constitution of lateral plates of mesoblast. Their splanchnic and somatic layers. Body-cavity constituting space between them. Their division into lateral and vertebral plates. Continuation of body-cavity into vertebral plates. Protovertebræ. Division into muscle-plates and vertebral bodies. Development of muscle-plates. Disappearance of segmentation in tissue to form vertebral bodies. Body-cavity and parietal plates. Primitive independent halves of body-cavity. Their ventral fusion. Separation of anterior part of body-cavity as pericardial cavity. Communication of pericardial and peritoneal cavities. Somatopleure and splanchnopleure. Résumé. General considerations on development of mesoblast. Probability of lateral plates of mesoblast in Elasmobranchii representing alimentary diverticula. Meaning of secondary segmentation of vertebral column. The urinogenital system.--Development of segmental duct and segmental tubes as solid bodies. Formation of a lumen in them, and their opening into body-cavity. Comparison of segmental duct and segmental tubes. Primitive ova. Their position. Their structure. The notochord.--The formation of its sheath. The changes in its cells.