670 REPRODUCTION. 



ually toward its circumference, returning upon themselves in loops, the 

 convexities of which are directed inward. The pupillary membrane 

 finally becomes atrophied, following in this process its receding blood- 

 vessels from the centre outward. It has completely disappeared by the 

 end of the seventh month. (Cruveilhier.) 



The eyelids are formed by folds of the integument, projecting from 

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

 during the second and third months that their margins come in contact 

 and adhere together, so that at that time they cannot be separated 

 without violence. They remain adherent until the seventh month 

 (Guy), when they again separate and become movable. In carnivorous 

 animals (dogs and cats), they remain adherent until eight or ten days 

 after birth. 



The internal ear is formed by an offshoot from the third cerebral 

 vesicle ; the passage of communication afterward filling up by a deposit 

 of white substance, which becomes the auditory nerve. The tympanum 

 and auditory meatus are derived from the external integument. 



Skeleton and Limbs. The spinal column makes its first appearance 

 as a series of cartilaginous rings deposited round the " chorda dor- 

 salis " (page 633). These rings, increasing in thickness by subsequent 

 growth, become ' the bodies of the vertebrae ; from which outgrowths 

 afterward take place in various directions, forming the transverse, 

 oblique, and spinous processes of the vertebra?, and enclosing the spinal 

 canal. 



When the union of the dorsal plates on the median line fails to 

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

 sents the malformation known as spina bifida. This may consist sim- 

 ply in a fissure of the spinal canal, more or less extensive, which may 

 sometimes be cured, or even close spontaneously ; or it may be com- 

 plicated with imperfect development or absence of the spinal cord at the 

 same spot, producing permanent paralysis of the lower limbs. 



The entire skeleton is at first cartilaginous. The earliest points of 

 ossification show themselves, about the beginning of the second month, 

 almost simultaneously in the clavicle and the lower jaw. Then come, 

 in the following order, the femur, humerus and tibia, the superior 

 maxilla, the bodies of the vertebrae, the ribs, the vault of the cranium, 

 the scapula and pelvis, the metacarpus and metatarsus, and the 

 phalanges of the fingers and toes. The bones of the carpus are all 

 cartilaginous at birth, and be*gin to ossify only at the end of the first 

 year. According to Cruveilhier, the calcaneum, the cuboid, and some- 

 times the astragalus, begin their ossification during the latter periods 

 of foetal life, but the remainder of the tarsus is cartilaginous at birth. 

 The lower extremity of the femur, according to the same authority, 

 shows a point of ossification at birth ; all the other extremities of the 

 long bones being still in a cartilaginous condition. The scaphoid bone 

 of the tarsus and the pisiform bone of the carpus are the last to com- 

 mence their ossification, several years after birth. Nearly all the bones 



