CHAPTER VI.

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.