These two sections shew that the heart is entirely formed from the mesoblast of the _splanchnopleure_, and that it is formed by the splitting of that part of the mesoblast which has turned to assume its normal direction after being folded in to form the muscular wall of the alimentary canal. In fig. 11 the cavities so formed on each side have not yet united, but in fig. 10 they have united. When the folding becomes more complete the cavities (_of_, _of_) in fig. 11 will unite, and in this way the origin of the omphalomeseraic veins will be carried further backwards. In the section immediately behind section 11 the mesoblast had become thickened, but had not split. EXPLANATION OF PLATES 3 AND 4. (V. p. 60.) 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.