594 WHITMAN. [Vot. II. 
many indications of a double origin, but the two vesicles in 
Petromyzon (Ahlborn, Beard), and the double pineal stalk of 
Varanus giganteus (Spencer), are facts that certainly favor such 
a view. Moreover, the position of the organ is such that we 
must suppose it to be derived from two sources, namely, the 
lateral (sutural) edges of the medullary plate. The fusion of 
segmental sense-organs has nothing improbable in it, as can be 
shown by more than one example from the Hzrudinea. On the 
other hand, the origin of organs by division, although claimed 
by a number of authors of high repute, has not a single verified 
fact in its favor. I am quite confident that such a process does 
not underlie the multiplication of lateral-line organs, and I have 
found nothing of the kind in the development of the segmental 
sense-organs of the Hzrudinea. 
The origin of the pineal eye from the lateral eyes, as held by 
Beard, is open to more serious objections. In the first place, it 
is difficult to believe that this organ is of later origin than the 
lateral eyes ; and in the second place, the idea that the entire 
optic vesicles (before and during the closure of the neural plate) 
represented retinal epithelium does not appear probable. 
Professor Cope has recently brought some new paleontological 
evidence to bear on this question, and suggests that the lateral 
eyes may have arisen from the pineal eye. The condition rep- 
resented in J/ycterops is not claimed to necessitate such a view, 
and there is another interpretation which seems to me admis- 
sible, and more in accordance with what we know of the genesis 
of sense-organs. If the whole median orifice is not to be re- 
garded as a parietal foramen, and if its lateral portions are really 
orbits as pointed out by Professor Cope, then there seems to be 
nothing in the way of interpreting the pair of orifices in the 
plate that divides the orbits as parietal foramina, if the possibil- 
ity of the double origin of the pineal eye be conceded. Beard 
has already called attention to the possible existence of a pari- 
etal foramen in a corresponding position in Asterolepis ornatus. 
Leydig’s interpretation of the pineal eye is the only one that 
approaches my view, but it does not carry the origin directly 
back to invertebrate segmental sense-organs. It must be re- 
membered, however, that Leydig! identifies the lateral-line 
1 Since the above was written, Professor Leydig has published a paper, in which 
he contends that the pineal organ is not an eye nor a sense-organ of any kind, but a 
