DEVELOPMENT OF THE ORGANS OF SENSE. 101 



has not yet been followed. The sympathetic can be seen as a knotted cord at 

 the end of the second month. 



The cerebral and spinal membranes are also, according to Kolliker, a pro- 

 duction from the protovertebral disks, and are recognizable about the sixth 

 week. As the fissures separating the parts of the cerebro-spinal axis appear, 

 the membranes extend down them, and the pia mater passes into the cerebral 

 ventricles. Bischoff, however, describes the pia mater and arachnoid as deve- 

 loped from the cerebral vesicles, and formed in the position which they perma- 

 nently occupy. 



Development of the Eye. The first rudiment of the eye is seen about the third 

 week, in a vesicle (primitive ocular vesicle), which communicates with the first 

 cerebral vesicle, and after the latter is divided into two, communicates with its 

 posterior division the Zwischenhirn or intermediary brain by a hollow 

 stalk, which afterwards becomes the optic nerve. This primitive ocular vesi- 

 cle, derived from the cerebral mass, is invested by a layer from the epidermic 

 lamina of the blastoderm ; from the latter layer are derived the conjunctiva, 

 the epithelium of the cornea, and the crystalline lens ; while the cephalic layer 

 gives origin to the vitreous body, the fibrous coat of the eye (sclerotic and 

 cornea), the choroid and iris, and the retina. 



The lens is formed by a thickening of the epidemic layer, opposite to the 

 primitive ocular vesicle, by which that vesicle is at first depressed, and then 

 reversed in the manner indicated by the annexed figures; so that the cavity of 

 the primitive ocular vesicle is finally obliterated. As this process takes place, 

 a secondary cavity (secondary ocular vesicle) is formed between the rudi- 

 mentary lens and the coats of the reversed primitive vesicle, and in this space 

 the vitreous humor is secreted. 



Diagram of development of the lens. ABC. Different stages of development. 1. Epidermic layer. 2. 

 Thickening of this layer. 3. Crystalline depression. 4. Primitive ocular vesicle, its anterior part pushed 

 back by the crystalline depression. 5. Posterior part of the primitive ocular vesicle, forming the external 

 layer of the secondary ocular vesicle. 6. Point of separation between the lens and the epidermic layer. 7. 

 Cavity of the secondary ocular vesicle, occupied by the vitreous. 



The lens is at first a mere depression in the epidermic layer. When this is 

 closed the lens becomes a vesicle, formed of epithelial cells, which grow and 

 fill its cavity, becoming gradually transformed into fibres. It is at first sur- 

 rounded by a vascular membrane the vascular capsule of the lens which is 

 connected with the termination of the temporary artery (hyaloid) that forms the 

 continuation of the central artery of the retina through the vitreous chamber. 

 This vascular capsule of the crystalline lens forms the membrane pupillaris 

 (described under the Anatomy of the Eye), and attaches the borders of the iris 

 to the capsule of the lens. It disappears about the seventh month. 



The sclerotic and cornea, except the epithelial layer of the latter, are formed 

 from the outer layer of the reversed primitive ocular vesicle, the retina from 

 the inner layer; the pigment of the choroid is also derived from the inner layer, 

 its proper tissue from one of these layers, but which has not yet been deter- 

 mined. The cavity of the primitive ocular vesicle disappears as that of the 

 optic nerve does. 



