Fig. 6. Horizontal section through a part of the choroid slit at stage P. Zeiss B, ocul. 2. The figure shews (1) the rudimentary processus falciformis (_pfal_) giving origin to the vitreous humour; and (2) the hyaloid membrane (_hym_) which is seen to adhere to the retina, and not to the vitreous humour or processus falciformis. EXPLANATION OF PLATE 18. (X. p. 446.) COMPLETE LIST OF REFERENCE LETTERS. _Nervous System._ _ar._ Anterior root of spinal nerve. _nc._ Neural canal. _pr._ Posterior root of spinal nerve. _spn._ Spinal nerve. _syg._ Sympathetic ganglion. _Alimentary Canal._ _al._ Alimentary canal. _alv._ Caudal vesicle of the postanal gut. _clal._ Cloacal section of alimentary canal. _du._ Duodenum. _hpd._ Ductus choledochus. _pan._ pancreas. _soes._ Solid oesophagus. _spv._ Intestine with rudiment of spiral valve. _umc._ Umbilical canal. _General._ _ao._ Dorsal aorta. _aur._ Auricle of heart. _cav._ Cardinal vein. _ch._ Notochord. _eppp._ Epithelial lining of the body-cavity. _ir._ Interrenal body. _me._ Mesentery. _mp._ Muscle-plate. _mpl_. Muscle-plate sending a prolongation into the limb. _po._ Primitive ovum. _pp._ Body-cavity. _sd._ Segmental duct. _st._ Segmental tube. _ts._ Tail swelling. _vcau._ Caudal vein. _x._ Subnotochordal rod. Fig. 1. Transverse section through the anterior abdominal region of an embryo of a stage between K and L. Zeiss B, ocul. 2. Reduced one-third. The section illustrates the junction of a sympathetic ganglion with a spinal nerve and the sprouting of the muscle-plates into the limbs (_mpl_). Fig. 2. Transverse section through the abdominal region of an embryo belonging to stage L. Zeiss B, ocul. 2. Reduced one-third. The section illustrates the junction of a sympathetic ganglion with a spinal nerve, and also the commencing formation of a branch from the aorta (still solid) which will pass through the sympathetic ganglion, and forms the first sign of the conversion of part of a sympathetic ganglion into one of the suprarenal bodies. Fig. 3. Longitudinal and vertical section of an embryo of a stage between L and M, shewing the successive junctions of the spinal nerves and sympathetic ganglia. Fig. 4. Section through the solid oesophagus during stage L. Zeiss A, ocul. 1. The section is taken through the region of the heart, so that the cavity of the auricle (_aur_) lies immediately below the oesophagus. Fig. 5. Optical section of the tail of an embryo between stages I and K, shewing the junction between the neural and alimentary canals. Fig. 6. Four sections through the caudal region of an embryo belonging to stage K, shewing the condition of the postanal section of the alimentary tract. Zeiss A, ocul. 2. An explanation of these figures is given on p. 449. Fig. 7. Section through the interrenal body of a Scyllium embryo belonging to stage Q. Zeiss C, ocul. 2.