STRUCTURE AND DEVELOPMENT OE LEPIDOSTEUS. 
383 
Sense Organs. 
Olfactory organ. 
Development. —The nasal sacs first arise during the late embryonic period in the 
form of a pair of thickened patches of the nervous layer of the epiblast on the dorsal 
surface of the front end of the head (Plate 24, fig. 39, ol.). The patches very soon 
become partially invaginated; and a small cavity is developed between them and the 
epidermic layer of the epiblast (Plate 24, figs. 42 and 43, ol.). Subsequently, the roof 
of this space, formed by the epidermic layer of the epiblast, is either broken through 
or absorbed ; and thus open pits, lined entirely by the nervous layer of the epidermis, 
are formed. 
We are not acquainted with any description of an exactly similar mode of origin of 
the olfactory pits, though the process is almost identical with that of the other sense 
organs. 
We have not worked out in detail the mode of formation of the double openings of 
the olfactory pits, but there can be but little doubt that it is caused by the division of 
the single opening into two. 
The olfactory nerve is formed very early (Plate 24, fig. 39,1), and, as Marshall has 
found in Aves and Elasmobranchii, it arises at a stage prior to the first differentiation 
of an olfactory bulb as a special lobe of the brain. 
The Eye , 
Anatomy .—We have not made a careful histological examination of the eye of 
Lepidosteus, which in our specimens was not sufficiently well preserved for such a 
purpose; but we have found a vascular membrane enveloping the vitreous humour 
on its retinal aspect, which, so far as we know, is unlike anything which has so far 
been met with in the eye of any other adult Vertebrate. 
The membrane itself is placed immediately outside the hyaloid membrane, i.e., on 
side of the hyaloid membrane bounding the vitreous humour. It is easily removed 
from the retina, to which it is only adherent at the entrance of the optic nerve. In 
both the eyes we examined it also adhered, at one point, to the capsule of the lens, but 
we could not make out whether this adhesion was natural, or artificially produced by 
the coagulation of a thin layer of albuminous matter. In one instance, at any rate, 
the adhesion appeared firmer than could easily be produced artificially. 
The arrangement of the vessels in the membrane is shown diagrammatically in 
Plate 25, fig. 49, while the characteristic form of the capillary plexus is represented in 
Plate 25, fig. 50. 
The arterial supply appears to be derived from a vessel perforating the retina close 
to the optic nerve, and obviously homologous with the artery of the processus falci- 
formis and pecten of Teleostei and Birds, and with the arteria centralis retinae of 
Mammals. From this vessel branches diverge and pursue a course towards the 
MDCCCLXXXII. 3 D 
