136 



DEVELOPMENT OF EYE 



as follows : The primary sympathetic cells are indifferent (ectodermic) elements which become 

 differentiated into two families of cells through a stage named in one case the sympaihoblast, 

 and in the other pMochromoblast (Pol '). which become respectively sympathetic nerve-cells and 

 chromophil or phaochrome cells. The significance of these researches in connexion with the 

 adrenal will be alluded to later. 



DEVELOPMENT OF THE EYE. 2 



The eyes begin to develop as a pair of hollow protrusions from the primitive 

 fore-brain, named the optic vesicles. In some mammals the protrusions appear 

 before the neural canal is closed in by the fusion of the medullary folds ; in the 



optic vesicle j&g 



optic vesicle 





rii pit ** 



- * 



1 1 it 1.1 r it -pit 



hind-brain 



FIG. 177. TRANSVERSE SECTION OF THE HEAD OF A 

 HUMAN EMBRYO OF 2'4 MM., SHOWING THE OPTIC 

 VESICLES AND AUDITORY PITS. (T. H. BryCG.) 



FIG. 178. SIDE VIEW or ANTERIOR 



PART OF BRAIN OP A HUMAN 

 EMBRYO OF THE FOURTH WEEK, 

 SHOWING THE PRIMARY OPTIC 

 VESICLE FOLDED AND CUPPED. 



(His.) 



c.h., cerebral hemisphere (part 

 of) ; off., olfactory lobe ; opt., optic 

 cup. 



FIG. 179. SIDE VIEW OF THE SAME PART 



OF THE BRAIN IN A STILL MORE AD- 

 VANCED EMBRYO, THE EYE HAVING 

 BEEN CUT AWAY. (His.) 



opt., cut end of optic stalk, showing 

 the manner in which it is folded; 

 i, infundibulum ; olf.p., posterior 

 of olfactory lobe ; olf.a., anterior part of 

 the same ; c.h., cerebral hemisphere ; 

 i.e., tuber cinereum. 



pig, for instance, as shown by Keibel, they show as shallow pits on the medullai 

 folds while these are still spread out flat. The appearances presented by the 

 very early human embryo drawn in fig. 177, show that this may occui 

 in the human subject also. The optic vesicle is continuous on its outer side with 

 the surface ectoderm of the side of the head ; and as this point of attachment does 

 not move so much during the formation of the cranial flexure as does the attach- 

 ment to the brain-tube, it follows that the vesicle becomes obliquely placed (His), 



1 In Hertwig's Haiidbuch der vergleich. Entwickelungslehre III. Th., for which see references to 

 recent literature. 



-' For literature, see Froriep, Hertwig's Handbuch II. Th. i. and li. p. 261 seq. More recent 

 references in footnotes. 



