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).