_al._ Alimentary tract. _an._ Point where anus will be formed. _ao._ Dorsal aorta. _ar._ Rudiment of anterior root of spinal nerve. _b._ Anterior fin. _c._ Connective-tissue cells. _cav._ Cardinal vein. _ch._ Notochord. _df._ Dorsal fin. _ep._ Epiblast. _ge._ Germinal epithelium. _ht._ Heart. _l._ Liver. _mp._ Muscle-plate. _mp´._ Early formed band of muscles from the splanchnic layer of the muscle-plates. _nc._ Neural canal. _p._ Protoplasm from yolk in the alimentary tract. _pc._ Pericardial cavity. _po._ Primitive ovum. _pp._ body-cavity. _pr._ Rudiment of posterior root of spinal nerve. _sd._ Segmental duct. _sh._ Cuticular sheath of notochord. _so._ Somatic layer of mesoblast. _sp._ Splanchnic layer of mesoblast. _spc._ Spinal cord. _sp.v._ Spiral valve. _sr._ Interrenal body. _st._ Segmental tube. _sv._ Sinus venosus. _ua._ Umbilical artery. _um._ Umbilical cord. _uv._ Umbilical vein. _V._ Splanchnic vein. _v._ Blood-vessel. _vc._ Visceral cleft. _Vr._ Vertebral rudiment. _W._ White matter of spinal cord. _x._ Subnotochordal rod (except in fig. 14_a_). _y._ Passage connecting the neural and alimentary canals. PLATE 11. Fig. 1. Section from the caudal region of a Pristiurus embryo belonging to stage H. Zeiss C, ocul. 1. Osmic acid specimen. It shews (1) the constriction of the subnotochordal rod (_x_) from the summit of the alimentary canal. (2) The formation of the body-cavity in the muscle-plate and the ventral thickening of the parietal plate. Fig. 1_a_. Portion of alimentary wall of the same embryo, shewing the formation of the subnotochord rod (_x_). Fig. 2. Section through the caudal vesicle of a Pristiurus embryo belonging to stage H. Zeiss C, ocul. 1. It shews the bilobed condition of the alimentary vesicle and the fusion of the mesoblast and hypoblast at the caudal vesicle. Fig. 3_a_. Sections from the caudal region of a Pristiurus embryo belonging to stage H. Zeiss C, ocul. 1. Picric acid specimen. It shews the communication which exists posteriorly between the neural and alimentary canals, and also by comparison with 3_b_ it exhibits the dilatation undergone by the alimentary canal in the caudal vesicle. Fig. 3_b_. Section from the caudal region of an embryo slightly younger than 3_a_. Zeiss C, ocul. 1. Osmic acid specimen. Fig. 4. Section from the cardiac region of a Pristiurus embryo belonging to stage H. Zeiss C, ocul. 1. Osmic acid specimen. It shews the formation of the heart (_ht_) as a cavity between the splanchnopleure and the wall of the throat. Fig. 5. Section from the posterior dorsal region of a Scyllium embryo, belonging to stage H. Zeiss C, ocul. 1. Osmic acid specimen. It shews the general features of an embryo of stage H, more especially the relations of the body-cavity in the parietal and vertebral portions of the lateral plate, and the early-formed band of muscle (_mp´_) in the splanchnic layer of the vertebral plate.