r.xi'i. \\.vi [ON 01 ri.vi xiii 



(4) The growth downwards of the muscle-plate to form the mtwcle* of the 

 abdomoL 



(5) The formal i..n of an outgi !e of the mesentery (/ ov), which 

 will become the < 



(/>) The spiral valve (a /). 



Fig. 14. Transparent view of the h< mbryo shortly Ixsfore the appear- 



1 the external gills. (M o diam.) Tin- chief point, tube n-.ticed are 



(1) The relation of the cranial nervi^ to the visceral clefts and the manner in 

 which the glosso-pharyngeal (g I) and v.i-us (v g) are united. 



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



(3) The eye (op). The stalk, as well as the bulb of ,-osed 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. em. Embryo. 

 ep. Kpiblast. f. Formative cells, by. Ilypoblast. / /. Lower layer cells. ///. 

 Mesoblast. M. Nuclei of yolk of Selachian egg. n c. Neural canal, s g. Segmenta- 

 tion 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 n, A m. Diagrammatic sections of Amphioxus in its early stages (founded 

 upon Kowalevsky's observations). 



B I, B II, r, III. Diagrammatic longitudinal sections of an hypothetical animal, 

 intermediate between Amphioxus and I'.atrachians, in its early stages. 



C I, C II, C in. Diagrammatic longitudinal sections of Bombinator igneus in its 

 early stages (founded upon (lotto's observation-). In c in the neural canal is i 

 pleted, which was not the case in i: in. The epibla-t in cm has been diagram- 

 mat ically represented as a single 1 



D I, D n, i) m. Diagrammatic longitudinal sections of an animal, intern, 

 between Batrachians and Selachian>. in its early stages. 



K i, E n, K m. Diagrammatic longitudinal sections of a Selachian in its early 

 stages. 



E'. Surface view of the yolk of a Selachian^ egg to shew the manner in which it 

 is enclosed by the bla-todmn. The yolk Ucd yellow and the bla>toderm 



blue. 



F I, F II, F III. Diagrammatic longitudin.;' id in its early stages. 



B. IV. 



