SKELETON AXD LIMBS. 641 



the centre toward the circumference. It has completely disappeared 

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



The eyelids are formed by folds of the integument, which 

 gradually project from above and below the situation of the eye- 

 ball. They grow so rapidly during the second and third months 

 that their free margins come in contact and adhere together, so that 

 they cannot be separated at that time without some degree of vio- 

 lence. They remain adherent from this period until the seventh 

 month (Guy), when their margins separate and they become per- 

 fectly free and movable. In the carnivorous animals, however 

 (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 both offshoots from the external integument. 



Skeleton. At a very early period of development there appears, 

 as we have already described (Chap. VII.), immediately beneath the 

 cerebro-spinal axis, a cylindrical cord, of a soft, cartilaginous con- 

 sistency, termed the chorda dorsalis. It consists of a fibrous sheath 

 containing a mass of simple cells, closely packed together and 

 united by adhesive material. This cord is not intended to be a 

 permanent part of the skeleton, but is merely a temporary organ 

 destined to disappear as development proceeds. 



Immediately around the chorda dorsalis there are deposited soon 

 afterward a number of cartilaginous plates, which encircle it in a 

 series of rings, corresponding in number with the bodies of the future 

 vertebrae. These rings increase in thickness from without inward, 

 encroaching upon the substance of the chorda dorsalis, and finally 

 taking its place altogether. The thickened rings, which have been 

 filled up in this way and solidified by cartilaginous deposit, become 

 the bodies of the vertebras ; while their transverse and articulating 

 processes, with the laminae and spinous processes, are formed by 

 subsequent outgrowths from the bodies in various directions. 



When the union of the dorsal plates upon the median line fails 

 to take place, the spinal canal remains open at that situation, and 

 presents the malformation known as spina bifida. This malforma- 

 tion may consist simply in a fissure of the spinal canal, more or less 

 extensive, in which case it may often be cured, or may even close 

 spontaneously ; or it may be complicated with an imperfect deve- 

 41 



