306 Charles R. Stockard 



optic cup. This eye occupies an almost ventro-median position 

 and is united to the brain by a soHd cellular stalk. Its contact 

 with ectoderm from which the lens will arise is not established as 

 the head-fold does not yet extend back to this point. An eye in such 

 a ventral position will oftentimes come in contact with the ecto- 

 derm at a later stage than would a normal lateral eye. Ordinary 

 iwo-eyed individuals at this age (forty-nine hours) were, like this 

 Cyclops, just beginning to form the optic cups and the lateral 

 ectoderm over the incipient cups showed a slight thickening, the 

 earliest indication of a lens. As a rule the cyclopean eye is some- 

 what slower than the normal in its rate of development and may 

 generally be compared with the eyes of slightly younger two- 

 eyed individuals. 



Several embryos at this age lack eyes entirely and belong to 

 the blind variety presently to be described. 



Two-eyed embryos when fifty-four hours old possess well- 

 formed optic cups and lenses still connected with the ectoderm, 

 although projecting into the cavity of the cup. The nasal pits 

 are clearly marked ectodermal invaginations in an anterior and 

 slightly median position relative to the eyes. The brain possesses 

 a well developed central cavity. A cyclopean eye of a distinctly 

 double composition from a fifty-four hour embryo is shown in 

 cross-section by Fig. 31. The optic cup is bilateral and two lens 

 anlagen are indicated by the thickened ventral ectoderm. The 

 section of the brain dorsal to this eye is small and hollow. It is a 

 portion of the diencephalon which is between a larger telen- 

 cephalon and a much larger mid-brain. This eye would finally 

 have produced a large median cyclopean organ of the double type 

 with two retinal areas and two lenses. Its connection with the 

 brain is through two closely approximated stalks and two optic 

 nerves would probably have formed later. Comparing such an 

 eye with that of Fig. 32, of like age we find that here the optic 

 cup is single and one lens is forming. Both sections show the 

 eye in practically similar positions. The embryo from which 

 Fig. 32 was taken possesses a well formed telencephalon and two 

 lateral nasal-pit thickenings of the anterior ectoderm. 



A horizontal section of a fifty- four hour double-eyed cyclops 



