Fig. 1. Section through the trunk of the embryo. Figs. 2-5. Sections through the neurenteric canal. Fig. B. Surface view of a somewhat older embryo than that from which Series A is taken. x 30. SERIES B. Sections through the embryo represented in Fig. B. x 120. Fig. 1. Section through the trunk of the embryo. Figs. 2, 3. Sections through the hind end of the medullary groove. Fig. 4. Section through the neurenteric canal. Fig. 5. Section through the primitive streak. Fig. C. Surface view of a somewhat older embryo than that represented in Fig. B. x 30. EXPLANATION OF PLATES 30, 31, 32. (XVII. p. 668.) PLATE 30. COMPLETE LIST OF REFERENCE LETTERS. _ch._ Cheliceræ. _ch.g._ Ganglion of cheliceræ. _c.l._ Caudal lobe. _p.c._ Primitive cumulus. _pd._ Pedipalpi. _pr.l._ Præoral lobe. _pp_{1}. _pp_{2}. _etc._ Provisional appendages. _sp._ Spinnerets. _st._ Stomodæum. I-IV. Ambulatory appendages. 1-16. Postoral segments. Fig. 1. Ovum, with primitive cumulus and streak proceeding from it. Fig. 2. Somewhat later stage, in which the primitive cumulus is still visible. Near the opposite end of the blastoderm is a white area, which is probably the rudiment of the procephalic lobe. Fig. 3_a_ and 3_b_. View of an embryo from the ventral surface and from the side when six segments have become established. Fig. 4. View of an embryo, ideally unrolled, when the first rudiments of the appendages become visible. Fig. 5. Embryo ideally unrolled at the stage when all the appendages have become established. Fig. 6. Somewhat older stage, when the limbs begin to be jointed. Viewed from the side. Fig. 7. Later stage, viewed from the side. Fig. 7_a_. Same embryo as fig. 7, ideally unrolled. Figs. 8_a_ and 8_b_. View from the ventral surface and from the side of an embryo, after the ventral flexure has considerably advanced. Fig. 9. Somewhat older embryo, viewed from the ventral surface. PLATES 31 AND 32. COMPLETE LIST OF REFERENCE LETTERS. _ao._ Aorta. _ab.g._ Abdominal nerve cord. _ch._ Cheliceræ. _ch.g._ Ganglion of cheliceræ. _ep._ Epiblast. _hs._ Hemispherical lobe of supra-oesophageal ganglion. _ht._ Heart. _l.l._ Lower lip. _m._ Muscles. _me._ Mesoblast. _mes._ Mesenteron. _mp.g._ Malpighian tube. _ms._ Mesoblastic somite. _oe._ OEsophagus. _p.c._ Pericardium. _pd._ Pedipalpi. _pd.g._ Ganglion of pedipalpi. _pr._ Proctodæum (rectum). _pr.c._ Primitive cumulus. _s._ Septum in abdomen. _so._ Somatopleure. _sp._ Splanchnopleure. _st._ Stomodæum. _su._ Suctorial apparatus. _su.g._ Supra-oesophageal ganglion. _th. g._ Thoracic ganglion. _v.g._ Ventral nerve cord. _y.c._ Cells derived from yolk. _yk._ Yolk. _y.n._ Nuclei of yolk cells. I_g_-IV_g_. Ganglia of ambulatory limbs. 1-16. Postoral segments.