14 
Proceedings of the Royal Society 
Explanation of Plates I. and II. 
In all the diagrams homologous layers are of the same colour. 
j Reference Letters. 
Bl. blastopore. 
D. B. Dorsal blood-vessel in ancestral subintestinal 
in actual vertebrate. 
Ep. Epiblast. 
Hy. Hypoblast. 
In. Infundibulum. 
Int. Intestine. 
K. v. Kupffer’s vesicle. 
Me. Mesoblast. 
Ne. Neurochord. 
Ne. c. Neural canal. 
No. Notochord. 
0. m. Original mouth. 
0. e. Original eye. 
Pe. Periblast. 
Fig. 1. Diagram of a transverse section of a herring embryo passing 
through Kupffer’s vesicle. The colouring of the layers is the only 
point in which this figure differs from the drawing of an actual 
section. 
Fig. 2. Diagram of a section through the plane of symmetry of a herring 
embryo at the same stage. This figure is drawn after the study of 
a series of transverse sections. 
Fig. 3. Diagram of a section through the plane of symmetry of an Elas- 
mobranch embryo when the blastopore and medullary groove are 
being closed. 
Fig. 4. Diagram of a section of a herring embryo through the region 
where Kupffer’s vesicle has given place to the intestine. 
Fig. 5. Diagram of a section through the plane of symmetry of an embryo 
of Petromyzon, in which the gastrula cavity exists as in Elasmo- 
branchs, but the neural canal is at first absent, as in Teleosteans. 
Fig. 6. Diagram of an ideal section of the Vertebrate ancestor ; the 
section is oblique, and passes through both eyes, and the original 
nearly terminal mouth, which in actual Vertebrates is represented 
by the infunbibulum. 
