Fig. 8. Portion of a section of the interrenal body of an adult Scyllium. Zeiss C, ocul. 2. EXPLANATION OF PLATE 19. (X. p. 460) COMPLETE LIST OF REFERENCE LETTERS. _Nervous System._ _n._ Nerve. _spn._ Spinal nerve. _syg._ Sympathetic ganglion. _Alimentary Canal._ _cl._ Cloaca. _incl._ Cloacal involution. _oeep._ OEsophageal epithelium. _pan._ Pancreas. _th._ Thyroid body. _General._ _abp._ Abdominal pocket (pore). _aur._ Auricle. _cav._ Cardinal vein. _cauv._ Caudal vein. _ly._ Lymphoid tissue. _mm._ Muscles. _od._ Oviduct. _pc._ Pericardium. _pp._ body-cavity. _sr._ Suprarenal body. _u._ Ureter. _vao._ Ventral aorta (anterior continuation of bulbus arteriosus). _ven._ Ventricle. _wd._ Wolffian duct. Figs. 1_a_, 1_b_, 1_c_. Three sections through the cloacal region of an embryo belonging to stage O. 1_a_ is the anterior of the three sections. Zeiss A, ocul. 2. Reduced one-third. 1_a_ shews the cloacal involution at its deepest part abutting on the cloacal section of the alimentary tract. 1_b_ is a section through a point somewhat behind this close to the opening of the Wolffian ducts into the cloaca. 1_c_ shews the opening to the exterior in the posterior part of the cloaca, and also the rudiments of the two abdominal pockets (_abp_). Fig. 2. Section through the cloacal region of an embryo belonging to stage P. Zeiss A, ocul. 2. The figure shews the solid anterior extremity of the cloacal involution. Fig. 3. Longitudinal vertical section through the thyroid body in a stage between O and P. Zeiss a a, ocul. 1. The figure shews the solid thyroid body (_th_) connected in front with throat, and terminating below the bulbus arteriosus. Fig. 4. Pancreas (_pan_) and adjoining part of the alimentary tract in longitudinal section, from an embryo between stages L and M. Zeiss A, ocul. 2. Fig. 5. Portion of liver network of stage L. Zeiss C, ocul. 2. The section is intended to illustrate the fact that the tubules or cylinders of which the liver is composed are hollow and not solid. Between the liver tubules are seen blood spaces with distinct walls, and blood corpuscles in their interior. Fig. 6. Section through part of one of the suprarenal bodies of an adult Scyllium hardened in chromic acid. Zeiss C, ocul. 2. The section shews the columnar cells forming the cortex and the more polygonal cells of the medulla. Fig. 7. Transverse section through the anterior suprarenal body of an adult Scyllium. Zeiss B, ocul. 2. Reduced one-third. The tissue of the suprarenal body has not been filled in, but only the sympathetic ganglion cells which are seen to be irregularly scattered through the substance of the body. The entrance of the nerve (_n_) is shewn, and indications are given of the distribution of the nerve-fibres.