1. Section through notochord immediately behind the head fold. 2. Section shewing medullary groove a little behind 1. 3. Section just in front of the primitive streak. 4 and 5. Sections through the front end of the primitive streak. FIG. L. Surface view of blastoderm with a very young primitive streak. EXPLANATION OF PLATES 46-53[6]. (XXV. p. 871.) Footnote 6: The explanations of the figures printed within inverted commas are by Professor Balfour, the rest are by the Editors. COMPLETE LIST OF REFERENCE LETTERS. A. Anus. _a._ Dorso-lateral horn of white matter in brain. _a.g._ Accessory gland of male (modified accessory leg gland). _at._ Antenna. _at.n._ Antennary nerve. _b._ Ventro-lateral horn of white matter of brain. _b.c._ Body-cavity. _bl._ Blastopore. C. Cutis. _c._ Postero-dorsal lobe of white matter of brain. _c.g._ Supra-oesophageal ganglia. _cl._ Claw. _c.m._ Circular layer of muscles. _co._ Commissures between the ventral nerve-cords. _co.2._ Second commissure between the ventral nerve-cords. _co_{1}. 2. Mass of cells developed on second commissure. _cor._ Cornea. _c.s.d._ Common duct for the two salivary glands. _cu._ Cuticle. _d._ Ventral protuberance of brain. _d.l.m._ Dorsal longitudinal muscle of pharynx. _d.n._ Median dorsal nerve to integument from supra-oesophageal ganglia. _d.o._ Muscular bands passing from the ventro-lateral wall of the pharynx at the region of its opening into the buccal cavity. E. Eye. E. Central lobe of white matter of brain. _e.n._ Nerves passing outwards from the ventral cords. _ep._ Epidermis. _ep.c._ Epidermis cells. F.1, F.2, _&c._ First and second pair of feet, &c. _f._ Small accessory glandular tubes of the male generative apparatus. F._g._ Ganglionic enlargement on ventral nerve-cord, from which a pair of nerves to foot pass off. _f.gl._ Accessory foot-gland. F._n._ Nerves to feet. _g.co._ Commissures between the ventral nerve-cords containing ganglion cells. _g.o._ Generative orifice. H. Heart. _h._ Cells in lateral division of body-cavity. _hy._ Hypoblast. _i.j._ Inner jaw. _j._ Jaw. _j.n._ Nerves to jaws. L. Lips. _l._ Lens. _l.b.c._ Lateral compartment of body-cavity. _le._ Jaw lever (cuticular prolongation of inner jaw lying in a backwardly projecting diverticulum of the buccal cavity). _l.m._ Bands of longitudinal muscles. M. Buccal cavity. M{1}. Median backward diverticulum of mouth or common salivary duct which receives the salivary ducts. _me._ Mesenteron. _mes._ Mesoblastic somite. _m.l._ Muscles of jaw lever. _m.s._ Sheets of muscle passing round the side walls of pharynx to dorsal body-wall. _od._ Oviduct. _oe._ OEsophagus. _oes.co._ OEsophageal commissures. _o.f.g._ Orifice of duct of foot-gland. _o.j._ Outer jaw. _op._ Optic ganglion. _op.n._ Optic nerve. _or.g._ Ganglionic enlargements for oral papillæ. _or.n._ Nerves to oral papillæ. _or.p._ Oral papillæ. _o.s._ Orifice of duct of segmental organ. _ov._ Ovary. P. Pads on ventral side of foot.