772 DEVELOPMENT OF THE NERVOUS SYSTEM. 



face, which becomes the retina ; the rest of its cavity being occupied 

 by a gelatinous substance, the vitreous body. The crystalline lens is 

 formed in a distinct follicle, which is an ofl'shoot of the integument, and 

 becomes partially imbedded in the anterior portion of the eyeball. The 

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

 what opaque until a late period of development. It becomes nearly 

 transparent a short time before birth. 



The iris is a muscular septum, formed in front of the crystalline lens. 

 Its central opening, which afterward becomes the pupil, is at first closed 

 by a vascular membrane, the pupillary membrane, passing across the 

 axis of the eye. The bloodvessels of this membrane, which are derived 

 from those of the iris, subsequently become atrophied. They disappear 

 first from its centre, and recede gradually toward its circumference ; 

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

 toward the centre. The pupillary membrane itself finally becomes atro- 

 phied, following in this retrograde process the direction of its receding 

 bloodvessels, namely, from the centre outward. It has completely dis- 

 appeared by the end of the seventh month. (Cruveilhier.) 



The eyelids are formed by folds of the integument, which project 

 from above and below at the situation of the eyeball. They grow so 

 rapidly during the second and third months that their free margins 

 come in contact and adhere together, so that at that time they cannot 

 be separated without some violence. They remain adherent from this 

 period until the seventh month (Guy), when their margins separate and 

 they become free and movable. In carnivorous animals (dogs and cats), 

 the eyelids do not separate from each other until eight or ten days after 

 birth. 



The internal ear is formed in a somewhat similar manner with the 

 eyeball, by an offshoot from the third cerebral vesicle ; the passage 

 between them filling up by a deposit of white substance, which becomes 

 the auditory nerve. The tympanum and auditory meatus are botli 

 offshoots from the external integument. 



Skeleton and Limbs. At a very early period of development there 

 appears, immediately beneath the medullary canal, a cylindrical cord, 

 termed the chorda dorsalis (page 725). It consists of a tubular sheath 

 containing a mass cf simple cells, closely packed together and united 

 by adhesive material. It does not become a permanent part of the 

 skeleton, but is a temporary organ destined to disappear as development 

 proceeds. 



On each side of the chorda dorsalis there is formed a series of rec- 

 tangular plates, the " primitive vertebrae," a portion of whose substance 

 is devoted to the formation of muscular tissue, while another portion 

 becomes the basis for the permanent vertebrae. The latter are de- 

 posited in the form of cartilaginous plates, which encircle the chorda 

 dorsalis in a series of rings, corresponding in number with the bodies 

 of the future vertebrae. The rings increase in thickness from without 

 inward, encroaching upon the substance of the chorda dorsalis, and 



