610 DEVELOPMENT OF THE EMBRYO. 



against each other in the median line, form the right and left hemi- 

 spheres, separated from each other by the longitudinal fissure. 

 A similar process of growth taking place in the spinal cord results 

 in the formation of the two lateral columns and the anterior and pos- 

 terior median fissures of the cord. Elsewhere the median fissure is 

 less complete, as, for example, between the two lateral halves of the 

 cerebellum, the two optic thalami and corpora striata, and the two 

 tubercula quadrigemina ; but it exists everywhere, and marks more 

 or less distinctly the division between the two sides of the nervous 

 centres, produced by the excessive growth of their lateral portions. 

 In this way the whole cerebro-spinal axis is converted into a double 

 organ, equally developed upon the right and left sides, and partially 

 divided by a longitudinal median fissure. 



Organs of Special Sense. The eyes are formed by a diverticulum 

 which grows out on each side from the first cerebral vesicle. This 

 diverticulum is at first hollow, its cavity communicating with that 

 of the hemisphere. Afterward, the passage between the two is filled 

 up with a deposit of nervous matter, and becomes the optic nerve. 

 The globular portion of the diverticulum, which is converted into 

 the globe of the eye, has a very thin layer of nervous matter depo- 

 sited upon its internal surface, which becomes the retina ; the rest 

 of its cavity being occupied by a gelatinous semi-fluid substance, 

 the vitreous body. The crystalline lens is formed in a distinct fol- 

 licle, which is an offshoot of the integument, and becomes partially 

 imbedded in the anterior portion of the globe of the eye. The 

 cornea also is originally a part of the integument, and remains 

 partially opaque until a very late period of development. Its tissue 

 clears up, however, and becomes perfectly transparent, shortly be- 

 fore birth. 



The iris is a muscular septum which is formed in front of the 

 crystalline lens, separating the anterior and posterior chambers of 

 the aqueous humor. Its central opening, which afterward becomes 

 the pupil, is at first closed by a vascular membrane, the pupillary 

 membrane, passing directly across the axis of the eye. The vessels 

 of this membrane, which are derived from those of the iris, subse- 

 quently become atrophied. They disappear first from its centre, 

 and afterward recede gradually toward its circumference ; returning 

 always upon themselves in loops, the convexities of which are directed 

 toward the centre of the membrane. The pupillary membrane itself 

 finally becomes atrophied and destroyed, following in this retro- 

 grade process the direction of its receding bloodvessels, viz., from 



