No. I.] OPTIC VESICLES OF ELASMOBRANCHS. Et 
expanded from its ancestral condition, and that certain sensory 
areas, originally lying outside, may have been included in it, 
does much to offset that objection. It is not improbable that 
there is neural plate material in the ear and other sensory 
organs of its same rank. 
Kupffer,! has shown in Petromyzon, that the ganglionic 
elements of the head region make two distinct rows of fusions 
with the epiblast. The upper set corresponds to the ganglionic 
sense organs, and the lateral line of comparative anatomy, and 
the lower, or epibranchial set, corresponds to the organs 
described by Beard under the name of Branchial sense 
organs. 
The observations I have recorded above afford evidence that 
the lateral eyes are derived from segmental organs — segmental 
in the sense that they occur in pairs serially arranged. Three 
pairs of these organs are clearly defined on the cephalic plate. 
The front pair is the optic vesicles, and the other two pairs I 
have called accessory optic vesicles. 
This condition implies an ancestral form possessing serially 
arranged eye-like sense organs. 
The significant question arises here : What is the relation of 
this segmental series containing the eye as its highest differ- 
entiation, to the lateral line series, including the ear as the 
example of its highest development? Although we have not 
sufficient data to return an entirely satisfactory answer, never- 
theless, the assumption that they are closely related is not 
without foundation. There certainly is no insurmountable 
objection to placing the eye and ear in parallel homologous 
series, one of which has become included in the expanded 
neural plate and the other has not. It is not even necessary 
to assume the independence of the two series. I think there 
is, to-day, really more evidence to support the view that they 
are genetically related than that they are entirely distinct from 
one another. 
MARINE BIOLOGICAL LABORATORY, 
Woops HOLL, Mass., July, 1893. 
1 Kupffer, Verh. Anat. Ges., Miinchen, 1891. 
