314 Charles R. Stockard 



part of one of the corpora bigemina is cut and the completely 

 single eye with its lens is found lying ventrally and in a median 

 position. The double olfactory pit is seen in front of the eye and 

 somewhat to one side of the head. The posterior part of the 

 section runs below the hind-brain and finally cuts it as the head 

 bends just in the middle region of the well formed auditory ves- 

 icles. The section thus presents the three sense organs, the single 

 Cyclopean eye, the nasal pits united into a double pit; the paired 

 ear vesicles alone are in their usual positions. 



A transverse section through the eye of a four day embryo is 

 illustrated in Fig. 42. The retina is unusually wide laterally 

 but no other indication of doubleness is shown. The choroid 

 coat is beginning to form and the eye is connected with the floor 

 of the brain by a single cellular stalk. The retina at this age is 

 only slightly diff'erentiated and there is no arrangement into 

 layers. This embryo has two distinct nasal plates. Several of 

 the Cyclopean fish show the nasal plates separate, although they 

 are usually represented by an anterior double plate near the 

 middle line. 



A nine day embryo of which Fig. 43 represents a section through 

 the eye has a finely developed brain, well expanded laterally and 

 perfect in general shape and structure. The eye is completely 

 single and the retina is partially formed into layers; the lens is 

 almost transparent and the vitreous humor is being formed about 

 it. The eye has all structures closely similar to those in a 

 paired eye of this age and would doubtless have functioned had 

 the embryo hatched. This specimen has a single nasal pit. 



Another cyclops of perfect structure when studied in sections 

 at thirteen days old showed the mouth posterior to the eye, hang- 

 ing as a ventral proboscis-like mass. Two nasal plates were 

 present and the eye was single. This eye. Fig. 44, was unusually 

 far forward and although the retina was well differentiated into 

 layers the humor had not perfectly formed behind the lens. The 

 small section of the brain is shown in Fig. 44 to be bilateral and 

 not unusual in appearance. Passing forward through the series 

 of sections to a place where the anterior end of the cyclopean eye- 

 ball stops, a minute lens is found lying in a ventro-median position, 



