MORPHOLOGICAL STUDIES. 
69 
fashion of figs. 3 and 16, PI. XI, of Heape's memoir of the 
Mole (No. 10). 
The parietal eye did not then exist (PI. VI, fig. a) . On the 
closure of the neural plate the eyes of course got shut in, and 
in order that no lens of the so-called Vertebrate type should 
be formed from the epiblast of the median neural line above the 
eyes, one must suppose that the median suture of the brain 
was not composed of nervous sensory epithelium like that of 
the retina (PI. VI, fig. b). A retinal epithelium of the median 
dorsal line could not degenerate to form a lens like that of 
Hatteria. It would be excited by the light, and a lens, if 
formed, would arise from the indifferent epiblast. A piece of 
ordinary nervous tissue, on the other hand, would degenerate 
into an epithelial structure. We have instances of that in the 
pallium of Teleostei and Ganoids ; and such a piece of tissue 
must be postulated in the median suture of the brain above 
the paired eyes. 
If this be granted, the development of the parietal eye as an 
apparently unpaired organ is easily explicable. After the 
closure, according to Balfour, Wiedersheim and others, in their 
phylogenetic development the paired eyes would receive light 
from two sources, through the skin of the lateral surface of 
the body, and through the suture of closure. As they grow 
towards the surface a portion of the retina of each of them 
still receives light through the suture, and it is this portion 
which forms the retina of the parietal eye. 1 Its lens is formed 
by the epithelium of the suture which we assume is not sensory. 
The process of this hypothetical development I have figured 
in the three diagrams in PI. VI. The way in which this 
subsidiary eye could be thus developed from part of the 
sensory epithelium of the paired eyes is strikingly exemplified 
in the actual facts of the development of Jacobson's organ in 
Reptiles from a portion of the olfactory epithelium. Of this 
development of Jacobson's organ I am preparing a memoir 
which will soon follow these lines. 
1 Thus, if no trace of the parietal eye now existed one could arrive at the 
conclusion that such must once have been the case by induction. 
