Mesenteron, pp. 754-774. Subnotochordal rod, pp. 754-756. Splanchnic mesoblast and mesentery, pp. 756-758. _Respiratory division of the Mesenteron_, pp. 758-766. Thyroid body, pp. 759-762. Thymus gland, pp. 762, 763. Swimming bladder and lungs, pp. 763-766, _The middle division of the Mesenteron_, pp. 766-771. Cloaca, pp. 766, 767. Intestine, pp. 767, 768. Liver, pp. 769, 770. Pancreas, pp. 770, 771. _Postanal section of the Mesenteron_, pp. 771-774. The stomodæum, pp. 774-778. Comparative development of oral cavity, pp. 774-776. Teeth, pp. 776-778. The proctodæum, pp. 778-780. Bibliography of alimentary canal, p. 780. EMBRYOLOGY. CHAPTER I. CEPHALOCHORDA. The developmental history of the Chordata has been studied far more completely than that of any of the groups so far considered; and the results which have been arrived at are of striking interest and importance. Three main subdivisions of this group can be recognized: (1) the Cephalochorda containing the single genus Amphioxus; (2) the Urochorda or Tunicata; and (3) the Vertebrata[1]. The members of the second and probably of the first of these groups have undergone degeneration, but at the same time the members of the first group especially undergo a less modified development than that of other Chordata. [1] The term Vertebrata is often used to include the Cephalochorda. It is in many ways convenient to restrict its use to the forms which have at any rate some indications of vertebræ; a restriction which has the further convenience of restoring to the term its original limitations. In the first volume of this work the term Craniata was used for the forms which I now propose to call Vertebrata. CEPHALOCHORDA. Our knowledge of the development of Amphioxus is mainly due to Kowalevsky (Nos. 1 and 2). The ripe eggs appear to be dehisced into the branchial or atrial cavity, and to be transported thence through the branchial clefts into the pharynx, and so through the mouth to the exterior. (Kowalevsky, No. 1, and Marshall, No. 5.) [FIG. 1. THE SEGMENTATION OF AMPHIOXUS. (Copied from Kowalevsky.) B. Stage with four equal segments. C. Stage after the four segments have become divided by an equatorial furrow into eight equal segments. D. Stage in which a single layer of cells encloses a central segmentation cavity. E. Somewhat older stage in optical section. _sg._ segmentation cavity.]