12 
J. T. CUNNINGHAM. 
and that the reason why it has such close relations with both 
neurochord and hypoblast is that its development in the indi- 
vidual has been thrown so far back, takes place so early, that 
the fusion between the three layers due to the influence of the 
primitive elongated blastopore has scarcely disappeared before 
the central part of the mesoblast is converted into the noto- 
chord. "We have got back then to the old idea of the homo- 
logy of the notochord and the three giant fibres beneath the 
nerve-cord in the earth-worm. 
The Origin of the Vertebrate Eye. — Prof. Balfour 1 
has pointed out that the eyes of Vertebrates like those of Crus- 
tacea develop as part of the thickening of epiblast which gives 
rise to the nervous system. Prof. Lankester 2 has inferred from 
the relations of the cerebral eye in Ascidians, that the ancestral 
Vertebrate was transparent and had eyes on the floor of the 
brain cavity. But Sedgwick’s revelation enables us to go a step 
further in tracing the evolution of the Vertebrate eye. In the 
ancestor of the Vertebrate before the neural canal had begun 
to form, two eyes existed somewhat in front and at the sides of 
the mouth actually in the region of the central nervous system. 
There is no impossible assumption in this; the eyes of Coelen- 
terates in the present day are in contact with the superficial 
nervous system, and probably eyes in nearly all cases existed 
in the same relation before the nervous system was separated 
from the epiblast. These eyes in the ancestral Vertebrate 
were open cups like those of a modern Patella, Haliotis, &c. 
When the nervous system formed a canal it covered over these 
eyes, which were then open to the neural canal, to the cavity 
of the anterior cerebral vesicle in the floor of which was the 
original mouth. The animal was probably at this time trans- 
parent, and light reached the simple eyes both through the 
roof of the cerebral vesicle and the sides of the head. Now, as 
the walls of the cerebral vesicle increased in thickness, and 
perhaps became more opaque than the rest of the body, the 
1 ‘Report of Brit. Ass. Meeting,’ 1880, “ Address to Department of Anat. 
and Pkys.,” Sec. D. 
“Degeneration,” ‘Nature,’ Series 1880. 
