EXPLANATION OF PLATES XIII, XIV & T^ —Continued. 



small bodies of cells closely applied to the upper and outer edge of the 

 neural canal. 



(2) The commencing formation of the cells which form the axial 

 skeleton from the inner (splanchnopleuric) layer of the muscle-plate. Sec- 

 tions b and c are given more especially to show the mode of formation of 

 the oviduct ipv). 



In b it is seen as a solid knob (ov), arising from the point vrhere the 

 somatopleure and splanchnopleure unite, and in c (the section behind b) 

 of a solid rod (ov) closely applied to the epiblast, which has grown back- 

 wards from the knob seen in b. 



N.B. — In all three sections only one side is completed. 



Fig. 12 a 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 (p w d), while b 

 is a section from the space between two involutions from the pleuro-peri- 

 toneal cavity, so that the Wolffian duct (at first solid) {w d) is not connected 

 as in a with the pleuro-peritoneal cavity. The further points shown in tiie 

 sections are — 



(1) The commencing formation of the spiral valve (a /). 



(2) The suprarenal body {su r). 



(3) The oviduct (ov), which lias acquired a lumen. 



(4) The increase in length of the muscle-plates, the spinal nerves, &c. 



Tig. 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 still continuing connections (primitive involutions) between 

 the Wolffian duct and the pleuro-peritoneal cavity (p w d). 



(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 (p ov), 

 which will become the ovary. 



(6) The spiral valve {a I). 



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 (ff I) and vagus {v g) are united. 



(2) The remnants of the pleuro-peritoneal cavity in the head {p p'). 



(3) The eye {op). The stalk, as well as the bulb of the eye, are sup- 

 posed to be in focus, so that the whole eye has a somewhat peculiar 

 appearance. 



