EXPLANATION OF PLATE 5. xiii 



(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 appear- 

 ance of the external gills. (Magnified 20 diam.) The chief points to be noticed are 

 (r) The relation of the cranial nerves to the visceral clefts and the manner in 

 which the glosso-pharyngeal (g 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 supposed 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. Epiblast. f. Formative cells, hy. Hypoblast. / /. Lower layer cells, m. 

 Mesoblast. . 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 ill. Diagrammatic sections of Amphioxus in its early stages (founded 

 upon Kowalevsky's observations). 



B I, B n, B in. Diagrammatic longitudinal sections of an hypothetical animal, 

 intermediate between Amphioxus and Batrachians, in its early stages. 



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

 early stages (founded upon Gotte's observations). In c III the neural canal is com- 

 pleted, which was not the case in B in. The epiblast in c in has been diagram- 

 matically represented as a single layer. 



D I, D u, D in. Diagrammatic longitudinal sections of an animal, intermediate 

 between Batrachians and Selachians, in its early stages. 



E i, E II, E III. Diagrammatic longitudinal sections of a Selachian in its early 

 stages. 



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

 is enclosed by the blastoderm. The yolk is represented yellow and the blastoderm 

 blue. 



F I, F n, F III. Diagrammatic longitudinal sections of a Bird in its early stages. 

 B. IV. C 



