Part 5

Fig. 12a and b. Two transverse sections of an embryo just before the appearance of the external gills. (Magnified 96 diam.)

In a there is seen to be an involution on each side (pwd), while b is a section from the space between two involutions from the pleuro-peritoneal cavity, so that the Wolffian duct (at first solid) (wd) is not connected as in a with the pleuro-peritoneal cavity. The further points shewn in the sections are--

(1) The commencing formation of the spiral valve (al). (2) The suprarenal body (sur). (3) The oviduct (ov), which has acquired a lumen. (4) The increase in length of the muscle-plates, the spinal nerves, &c.

Fig. 13. Section through the dorsal region of an embryo in which the external gills are of considerable length. (Magnified 40 diam.) The chief points to be noticed:

(1) The formation of the Wolffian body by outgrowths from the Wolffian duct (wd). (2) One of the still continuing connections (primitive involutions) between the Wolffian duct and the pleuro-peritoneal cavity (pwd). (3) The oviduct largely increased in size (ov). N.B. On the left side the oviduct has been accidentally made too small. (4) The growth downwards of the muscle-plate to form the muscles of the abdomen. (5) The formation of an outgrowth on each side of the mesentery (pov), which will become the ovary. (6) The spiral valve (al).

Fig. 14. Transparent view of the head of an embryo shortly before the appearance of the external gills. (Magnified 20 diam.) The chief points to be noticed are--

(1) The relation of the cranial nerves to the visceral clefts and the manner in which the glosso-pharyngeal (gl) and vagus (vg) are united. (2) The remnants of the pleuro-peritoneal cavity in the head (pp). (3) The eye (op). The stalk, as well as the bulb of the eye, are supposed to be in focus, so that the whole eye has a somewhat peculiar appearance.

EXPLANATION OF PLATE 5. (VI. p. 112.)

COMPLETE LIST OF REFERENCE LETTERS.

al. Cavity of alimentary canal. bl. Blastoderm. ch. Notochord. ep. Epiblast. em. Embryo. f. Formative cells. hy. Hypoblast. ll. Lower layer cells. m. Mesoblast. n. Nuclei of yolk of Selachian egg. nc. Neural canal. sg. Segmentation cavity. x. Point where epiblast and hypoblast are continuous at the mouth of the alimentary involution. This point is always situated at the tail end of the embryo. yk. Yolk.

Epiblast is coloured blue, mesoblast red, and hypoblast yellow. The lower layer cells before their separation into hypoblast and mesoblast are also coloured green.

A I, A II, A III. Diagrammatic sections of Amphioxus in its early stages (founded upon Kowalevsky's observations).

B I, B II, B III. Diagrammatic longitudinal sections of an hypothetical animal, intermediate between Amphioxus and Batrachians, in its early stages.

C I, C II, C III. Diagrammatic longitudinal sections of Bombinator igneus in its early stages (founded upon Götte's observations). In C III the neural canal is completed, which was not the case in B III. The epiblast in C III has been diagrammatically represented as a single layer.

D I, D II, D III. Diagrammatic longitudinal sections of an animal, intermediate between Batrachians and Selachians, in its early stages.

E I, E II, E III. Diagrammatic longitudinal sections of a Selachian in its early stages.

E´. Surface view of the yolk of a Selachian's egg to shew the manner in which it is enclosed by the Blastoderm. The yolk is represented yellow and the Blastoderm blue.

F I, F II, F III. Diagrammatic longitudinal sections of a Bird in its early

EXPLANATION OF PLATE 6. (X. p. 222.)

Fig. 1. Section through the germinal disc of a ripe ovarian ovum of the Skate. gv. germinal vesicle.

Fig. 2. Surface-view of a germinal disc with two furrows.

Figs. 3, 4, 5. Surface-views of three germinal discs in different stages of segmentation.

Fig. 6. Section through the germinal disc represented in fig 3. n. nucleus; x. edge of germinal disc. The engraver has not accurately copied my original drawings in respect to the structure of the segmentation furrows.

Figs. 6a and 6b. Two furrows of the same germinal disc more highly magnified.

Fig. 6c. A nucleus from the same germinal disc highly magnified.

Fig. 7. Section through a germinal disc of the same age as that represented in fig. 4. n. nucleus; nx. modified nucleus; nx´. modified nucleus of the yolk; f. furrow appearing in the yolk around the germinal disc.

Figs. 7a, 7b, 7c. Three segments with modified nuclei from the same germinal disc.

Fig. 8. Section through a somewhat older germinal disc. ep. epiblast; n´. nuclei of yolk.