DEVELOPMENT OF THE ELASMOBRANCH FISHES. I I I 



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

 and splachnopleure unite, and in c (the section behind l>) as a solid rod (ov) closely ap- 

 plied to the epiblast, which has grown backwards from the knob seen in b. 



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



Fig. 120 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-peritoneal 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 shewn in the sections are 



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



(2) The supra- renal body (si/ >). 



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



(4) The increase in length of the muscle-plates, the spinal nerve 1 :, &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 (w d). 



(2) One of the still continuing connections (primitive involutions) between 

 the Wolffian duct and the pleuro-peritoneal cavity (/ 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. 



(?) The formation of an outgrowth on each side of the mesentery (/ ov), which 

 will become the ovary. 



(6) The spiral valve (a /). 



Fig. 14. Transparent view of the head of an embryo shortly before the appear- 

 ance 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 (g /) and vagus (v g) are united. 



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



(3) The eye (of). 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. 



