Part 29

SERIES C, 1 and 2. Sections through a blastoderm with a young primitive streak.

1. Section through the front end of the primitive streak. 2. Section through the primitive streak, somewhat behind 1. Both figures shew very clearly the difference in character between the cells of the epiblastic mesoblast of the primitive streak, and the more granular cells of the mesoblast derived from the hypoblast.

FIG. D. Longitudinal section through the axial line of the primitive streak, and the part of the blastoderm in front of it, of an embryo duck with a well-developed primitive streak.

PLATE 44.

SERIES E, 1, 2, 3 and 4. Sections through blastoderm with a primitive streak, towards the end of the first stage.

1. Section through the anterior part of the area pellucida. 2. Section a little way behind 1 shewing a forward growth of mesoblast from the primitive streak. 3. Section through primitive streak. 4. Section through posterior part of primitive streak, shewing the great widening of primitive streak behind.

SERIES F, 1 and 2. Sections through a blastoderm with primitive groove.

1. Section shewing a deep pit in front of primitive streak, probably an early indication of the neurenteric canal. 2. Section immediately following 1.

FIG. G. Section through blastoderm with well-developed primitive streak, shewing an exceptionally deep slit-like primitive groove.

SERIES H, 1 and 2. Sections through a blastoderm with a fully-developed primitive streak.

1. Section through the anterior part of area pellucida, shewing the cubical granular hypoblast cells in this region. 2. Section slightly behind 1, shewing the primitive hypoblast cells differentiated into stellate cells, which can hardly be resolved in the middle line into hypoblast and mesoblast.

PLATE 45.

SERIES I, 1, 2, 3, 4 and 5. Sections through blastoderm somewhat older than Series H.

1. Section through area pellucida well in front of primitive streak. 2. Section through area pellucida just in front of primitive streak. 3. Section through the front end of primitive streak. 4. Section slightly behind 3. 5. Section slightly behind 4.

SERIES K, 1, 2, 3, 4 and 5. Sections through a blastoderm in which the first traces of notochord and medullary groove have made their appearance. Rather more than half the section is represented in each figure, but the right half is represented in 1 and 3, and the left in 2 and 4.

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. (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. p. Common duct into which the vasa deferentia open. p.c. Posterior lobe of brain. p.d.c. Posterior commissure passing dorsal to rectum. p.f. Internal opening of nephridium into body-cavity. ph. Pharynx. pi. Pigment in outer ends of epidermic cells. pi.r. Retinal pigment. p.n. Nerves to feet. p.p. Primary papilla. pr. Prostate. R. Rectum. Re. Retinal rods. R. m. Muscle of claw. s. Vesicle of nephridium. s{1}. Part of 4th or 5th nephridium which corresponds to vesicle of other nephridia. s.c.1. Region No. 1 of coiled tube of nephridium. s.c.2. Region No. 2 of ditto. s.c.3. Region No. 3 of ditto. s.c.4. Region No. 4 of ditto. s.d. Salivary duct. s.g. Salivary gland. sl.d. Reservoir of slime gland. sl.g. Tubules of slime gland. s.o.1, 2, 3, &c. Nephridia of 1st, 2nd, &c., feet. s.o.f. Terminal portion of nephridium. s.p. Secondary papilla. st. Stomach. st.e. Epithelium of stomach. sy. Sympathetic nerve running in muscles of tongue and pharynx. sy´. Origin of pharyngeal sympathetic nerves. T. Tongue. t. Teeth on tongue. te. Testis. tr. Tracheæ. tr.c. Cells found along the course of the tracheæ. tr.o. Tracheal stigma. tr.p. Tracheal pit. ut. Uterus. v.c. Ventral nerve cord. v.d. Vas deferens. v.g. Imperfect ganglia of ventral cord.

PLATE 46.

Fig. 1. Peripatus capensis, × 4; viewed from the dorsal surface. (From a drawing by Miss Balfour.)

PLATE 47.

Fig. 2. A left leg of Peripatus capensis, viewed from the ventral surface; x 30. (From a drawing by Miss Balfour.)

Fig. 3. A right leg of Peripatus capensis, viewed from the front side. (From a drawing by Miss Balfour.)

Fig. 4. The last left (17th) leg of a male Peripatus capensis, viewed from the ventral side to shew the papilla at the apex of which the accessory gland of the male, or enlarged crural gland, opens to the exterior. (From a drawing by Miss Balfour.) Prof. Balfour left a rough drawing (not reproduced) shewing the papilla, to which is appended the following note. "Figure shewing the accessory genital gland of male, which opens on the last pair of legs by a papilla on the ventral side. The papilla has got a slit-like aperture at its extremity."

Fig. 5. Ventral view of head and oral region of Peripatus capensis. (From a drawing by Miss Balfour.)