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. Fig. 6. Section from the oesophageal region of Scyllium embryo belonging to stage I. Zeiss C, ocul. 1. Chromic acid specimen. It shews the formation of the rudiments of the posterior nerve-roots (_pr_) and of the vertebral rudiments (_Vr_). Fig. 7. Section of a Torpedo embryo belonging to stage slightly later than I. Zeiss C, ocul. 1, reduced 1/3. Osmic acid specimen. It shews (1) the formation of the anterior and posterior nerve-roots. (2) The solid knob from which the segmental duct (_sd_) originates. Fig. 8. Section from the dorsal region of a Scyllium embryo belonging to a stage intermediate between I and K. Zeiss C, ocul. 1. Chromic acid specimen. It illustrates the structure of the primitive ova, segmental tubes, notochord, etc. Fig. 8_a_. Section from the caudal region of an embryo of the same age as 8. Zeiss A, ocul. 1. It shews (1) the solid oesophagus. (2) The narrow passage connecting the pericardial (_pc_) and body cavities (_pp_). Fig. 9. Section of a Pristiurus embryo belonging to stage K. Zeiss A, ocul. 1. Osmic acid specimen. It shews the formation of the liver (_l_), the structure of the anterior fins (_b_), and the anterior opening of the segmental duct into the body-cavity (_sd_). Figs. 9_a_, 9_b_, 9_c_, 9_d_. Four sections through the anterior region of the same embryo as 9. Osmic acid specimens.