SIGNIFICANCE OF KUPFFER’s VESICLE. 
13 
eyes would be affected in greater proportion by the light 
passing through the side of the head. For this reason the eye 
cups would deepen and begin to grow out from the cerebral 
vesicle in order (as already pointed out by Lankester) to 
get nearer to the lateral surface of the head. They would 
ultimately come into contact with the epiblast at this surface. 
The part of the epiblast with which the optic cups came 
into contact has remained transparent while the rest of the 
body has become opaque. The reason why the crystalline 
lens is now formed as an invagination is easily explained. 
The epiblast at this region became thickened in order to 
act as a lens. The lens was ultimately perfected by being 
converted into the biconvex shape and removed from the body 
surface. The separation of the retina from the crystalline 
lens is also comprehensible, for it was an improvement of the 
optical apparatus at every step. 
I do not intend here to discuss the rationale of the origin 
of the new mouth, of the visceral clefts, or of the actual anus. 
Although I believe these to be new structures I am not satis- 
fied as to their explanation. Sedgwick regards visceral clefts 
as serially homologous with segmental organs, but I have not 
considered the subject sufficiently at present to have a definite 
opinion. I will conclude with a few words as to segmentation. 
Sedgwick’s interesting theory does not at all explain the 
peculiarity of the growth of segmental animals, namely by the 
formation of new segments between the last and the end of the 
body. But this mode of growth is another difficulty in the 
way of his view of the blastopore. For he supposes the planes 
of segmentation to be originally perpendicular to the direction 
of the blastopore, and yet if the actual Vertebrate anus is the 
end of the blastopore, the planes of segmentation in the Verte- 
brate tail are parallel to the direction of the blastopore. 
