EXPLANATION OF PLATES 3 AND 4. COMPLETE LIST OF REFERENCE LETTERS. _al._ Alimentary canal. _ao._ Dorsal aorta. _auv._ Auditory vesicle. _bd._ Formative cell probably derived from the yolk. _cav._ Cardinal vein. _ch._ Notochord. _ch´._ Thickening of hypoblast to form the notochord. _eb._ Line indicating the edge of the blastoderm. _ep._ Epiblast. _ep´._ Epidermis. _er._ Embryonic rim. _es._ Embryonic swelling. _gl._ Glosso-pharyngeal nerve. _h._ Head. _ht._ Heart. _hy._ Hypoblast. _ll._ Lower layer cells. _ly._ Line of separation between the blastoderm and the yolk. _m._ Mesoblast. _mc._ Medullary canal. _mg._ Medullary groove. _mp._ Muscle-plate. _mp´._ Early formed mass of muscles. _n._ Peculiar nuclei formed in the yolk. _n´._ Similar nuclei in the cells of the blastoderm. _na._ Cells which help to close in the alimentary canal, and which are derived from the yolk. _ny._ Network of lines present in the food-yolk. _ol._ Olfactory pit. _op._ Eye. _ov._ Oviduct. _pn._ Pineal gland. _pov._ Projection which becomes the ovary. _pp._ Pleuro-peritoneal cavity. _pp´._ Remains of pleuro-peritoneal cavity in the head. _prv._ Protovertebræ. _pwd._ Primary points of involution from the pleuro-peritoneal cavity by the coalescence of which the Wolffian duct is formed. _sg._ Segmentation cavity. _so._ Somatopleure. _sos._ Stalk connecting embryo with yolk-sac. _sp._ Splanchnopleure. _spn._ Spinal nerve. _sur._ Suprarenal body. _ts._ Caudal lobes. _v._ Blood-vessel. _vg._ Vagus nerve. V. Fifth nerve. VII. Seventh nerve. _vc_, 1, 2, 3, &c. 1st, 2nd and 3rd &c. visceral clefts. _vp._ Vertebral plates. _wd._ Wolffian duct. _x._ Peculiar body underlying the notochord derived from the hypoblast. _yk._ Yolk spherules. All the figures were drawn with the Camera Lucida. Plate 3. Fig. 1. Section parallel with the long axis of the embryo through a blastoderm, in which the floor of the segmentation cavity (_sg_) is not yet completely lined by cells. The roof of the segmentation cavity is broken. (Magnified 60 diam.) The section is intended chiefly to illustrate the distribution of nuclei (_n_) in the yolk under the blastoderm. One of the chief points to be noticed in their distribution is the fact that they form almost a complete layer under the floor of the segmentation cavity. This probably indicates that the cells whose nuclei they become take some share in forming the layer of cells which subsequently (vide fig. 4) forms the floor of the cavity. Fig. 2. Small portion of blastoderm and subjacent yolk of an embryo at the time of the first appearance of the medullary groove. (Magnified 300 diam.)