The sections shew the features of various parts of the embryo. Figs. 3_a_, 3_b_ and 3_c_ belong to the head, and special attention should be paid to the presence of a cavity in the mesoblast in 3_b_ and to the ventral curvature of the medullary folds. Fig. 3_d_ belongs to the neck, fig. 3_e_ to the back, and fig. 3_f_ to the tail. Fig. 4. Section through the region of the tail at the commencement of stage F. (× 60 diameters.) The section shews the character of the tail-swellings and the commencing closure of the medullary groove. Fig. 5. Transverse section through the anterior part of the head of an embryo belonging to stage F (× 60 diameters). It shews (1) the ventral curvature of the medullary folds next the head. (2) The absence of mesoblast in the anterior part of the head. _hy_ points to the extreme front end of the alimentary canal. Fig. 6. Section through the head of an embryo at a stage intermediate between F and G. (× 86 diameters.) It shews the manner in which the medullary folds of the head unite to form the medullary canal. Fig. 7. Longitudinal and vertical section through the tail of an embryo belonging to stage G. It shews the direct communication which exists between the neural and alimentary canals. The section is not quite parallel to the long axis of the embryo, so that the protovertebræ are cut through in its anterior part, and the neural canal passes out of the section anteriorly. Fig. 8. Network of nuclei from the yolk of an embryo belonging to stage H. EXPLANATION OF PLATES 11 AND 12. (X. p. 315.) COMPLETE LIST OF REFERENCE LETTERS. _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.