xii EXPLANATION OF PLATE 4. 



The two caudal lobes, and the very marked groove between them, are seen at t s. 

 The front end of the notochord became indistinct, and I could not see its exact 

 termination. The epithelium of the alimentary canal (a I) is seen closely underlying 

 the notochord and becoming continuous with the epiblast at the hind end of the 

 notochord. 



The first visceral cleft (i v c) and eye (op) are just commencing to be formed, and 

 the cranial flexure has just appeared. 



Fig. 10. Section through the dorsal region of an embryo somewhat older than 

 the one represented in fig. 9. (Magnified 96 diam.) 



It shews (i) the formation by a pinching off from the top of the alimentary canal of 

 a peculiar body which underlies the notochord (x); (2) the primitive extension of the 

 pleuro-peritoneal cavity up to the top of the vertebral plates. 



PLATE 4. 



Fig. ii a, b, and c. Three sections closely following each other from an embryo 

 in which three visceral clefts are present; a is the most anterior of the three. (Mag- 

 nified 96 diam.) In all of these the muscle-plates are shewn at m p. They have 

 become separated from the lateral plates in b and c, but are still continuous with them 

 in a. The early formed mass of muscles is also shewn in all the figures (m p'}. 



The figures further shew (i) the formation of the spinal nerves (sp n) as 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. Sections b and c are given 

 more especially to shew the mode of formation of the oviduct (ov). 



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

 and splanchnopleure unite, and in c (the section behind b) as a solid rod (ov) closely 

 applied 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. 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 -M 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 /). 



(2) The supra- renal body (/ r). 



(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 (w d). 



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

 the Wolfnan 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. 



