_General features of the epiblast._--Original uniform constitution. Separation into lateral and central portions. _The medullary groove._--Its conversion into the medullary canal. _The mesoblast._--Its division into somatic and splanchnic layers. Formation of protovertebræ. The lateral plates. The caudal swellings. The formation of the body-cavity in the head. _The alimentary canal._--Its primitive constitution. The anus of Rusconi. Floor formed by yolk. Formation of cellular floor from cells formed around nuclei of the yolk. Communication behind of neural and alimentary canals. Its discovery by Kowalevsky. Its occurrence in other instances. _General features of the hypoblast._ _The notochord._--Its formation as a median thickening of the hypoblast. Possible interpretations to be put on this. Its occurrence in other instances. 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. CHAPTER VII. GENERAL DEVELOPMENT OF THE TRUNK FROM STAGE K TO THE CLOSE OF EMBRYONIC LIFE, pp. 361-377.