12 



THE PINEAL ORGAN 



sensitive part of the retina, does not appear at this stage of development 

 to differ markedly from that of the lens placode. 



The invagination of the outer part of the primary optic vesicle into 

 the ensheathing segment which becomes the pigment layer of the retina 

 constitutes the essential structural difference between the " upright eye," 

 typical of invertebrates, and the " inverted " eye which is characteristic 



Op.Cr.#-J^-- 



S.Med. 



Fig. 7. — Dorsal View of the Head Region of a Human Embryo at an Early 

 Stage of Development, showing the Optic Grooves before the Closure 

 of the Neural Tube. (Legge embryo, 12 somites, after Bartelmez and 



Evans.) 



Am. : Cut edge of Amnion. 



Ch. : Site of the future optic chiasma, where the right and left halves 



of the primordium of the optic crest become continuous anteriorly. 

 M.B. : Midbrain. 

 Op. Cr. : Stippled area representing the region beneath which the cells 



of the optic crest originate. 

 Op. S. : Optic groove. 

 P.O. Cr. : Primordium opto-cristale. 

 Rh. A. : Anterior segment of Rhombencephalon. 

 5. Cr. : Sulcus cristale. 

 5. Med. : Sulcus medullaris. 

 S.18 : Arrows indicating the plane of section Fig. 8. 



of the paired lateral eyes of all vertebrates. In the inverted eye the 

 receptive end of the sensory retinal cells — rods and cones — is turned away 

 from the source of light, entering the eye through the pupil. In those 

 animals, however, such as the horse and ox, in which a tapetum lucidum 

 is present on the retinal surface of the choroid coat and gives to this a 

 metallic lustre, it is believed that light is reflected by it on to the outer 



