20 Dynamic TJieory. 



rather long bladder-like swelling takes place, which is the first intima- 

 tion of a brain. Soon this long bladder brain is partly separated into 

 three parts by two constrictions. These parts are called the "fore- 



FIG. 25 Primitive skull 

 of human embryo of four 

 weeks vertical longitud- 

 inal section through the 

 middle. 



v z m h n The five sec- 

 tions of the skull cavity 

 occupied by the five brain 

 bladders respectively; 

 viz., the fore-brain, twixt- 



brain, mid-brain, hind- 

 brain and after-brain. 



o. Ear vesicle. 



a. Eye. 



no. Optic nerve. 



p. Canal of pituitary 

 gland. 



t Central part of the 

 crania] basis. 



(Kolliker.) 



brain," "mid-brain, " and ' 'hind-brain. " A little later and a fourth and 

 a fifth segment of brain bladder is formed by the division of the ' 'fore- 

 brain" and "hind-brain" into two each. There are, then, five brain 

 bladders, the first of which in the human subject, develops into the 

 cerebrum and olfactory lobes, the second into the optic thalamus with 

 the third ventricle, the third into the corpora quadrigemina or optic 

 lobes, the fourth into the cerebellum, and the last into the medulla ob- 

 longata or upper enlargement of the spinal cord. 



(See chapters on the brain.) 



In the meantime the eyes have been started by a pear-shaped vesicle 

 pushing out on each side from the third ventricle of the brain, against 

 the outer skin. These vesicles have each a hollow stalk or stem reach- 

 ing back to the cavity of the "twixt-brain" bladder of third ventricle. 

 The outer skin is first protruded by the eye vesicle pushing out. It 

 thickens up over the eye vesicle, then an indentation from without is 

 formed in the thickened skin. This deepens into a little cup, the edges 

 of which contract toward each other until they touch, and so form a ball. 

 This is the future lens of the eye. The formation of this lens in this 

 manner has the effect to push in and double back on itself, the front end 

 of the vesicle forming a double skinned cup, fitting the lens. The in- 

 side skin of this cup next the lens represents what afterward becomes 

 the retina. The outer layer of the cup (next the brain) represents the 

 pigment membrane which gives the eye its dark color. A fold from 

 the "flesh stratum" now forces its way from below the eye, inside of 

 the skin stratum, and enters between the lens and retina and forms the 

 rudiment of the transparent gelatinous body called the vitreous humor 

 of the eye. A strip of this vitreous body also pushes its way along the 

 underside of the first formed stem or eye stalk, the rudimentary optic 

 nerve. This stalk thereupon first collapses 'into a double layered semi- 

 cylindrical strap. It then folds itself over the strip of vitreous matter 

 thus forming a new tube, second rudimentary optic nerve, with the vit- 



