134 EARLY STAGES IN DEVELOPMENT OF VERTEBRATES. 



EXPLANATION OF PLATE 5. 



COMPLETE LIST OF REFERENCES. 



al. Cavity of alimentary canal, bl. Blastoderm, ch. Notochord. ep. Epiblast. 

 em. Embryo, f. Formative cells, hy. Hypoblast. / /. Lower layer cells. i. 

 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 II, A ill. Diagrammatic sections of Amphioxus in its early stages (founded 

 upon Kowalevsky's observations). 



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

 intermediate between Amphioxus and Batrachians, in its early stages. 



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

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

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

 matically represented as a single layer. 



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

 between Batrachians and Selachians, in its early stages. 



E I, E II, E ill. 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 II, F ill. Diagrammatic longitudinal sections of a Bird in its early stages. 



