762 GENERATION AND DEVELOPMENT. 



tlie stylo-liyoid ligament, and the smaller cornu of the hyoid 

 bone. From the third visceral arch, the greater cornu and 

 body of the hyoid bone. In man and other mammalia the 

 fourth visceral arch is indistinct. 



Development of the Extremities. 



The extremities are developed in an uniform manner ia 

 all vertebrate animals. They appear in the form of leaf- 

 like elevations from the parietes of the trunk (see fig. 

 231), at points where more or less of an arch will be pro- 

 duced for them within. The primitive form of the ex- 



Fiy- 231-* 



tremity is nearly the same in all Vertebrata, whether it be 

 destined for swimming, crawling, walking, or flying. In 

 the human foetus the fingers are at first united, as if 

 webbed for swimming ; but this is to be regarded not so 



* Fig. 231. A human embryo of the fourth week, 3 4 lines in length. 

 i, the chorion ; 3, part of the amnion ; 4, umbilical vesicle with its long 

 pedicle passing into the abdomen ; 7, the heart ; 8, the liver ; 9, the 

 visceral arch destined to form the lower jaw, beneath which are two 

 other visceral arches separated by the branchial clefts ; 10, rudiment of 

 the upper extremity ; 1 1, that of the lower extremity ; 12, the umbilical 

 cord; 15, the eye ; 16, the ear; 17, the cerebral hemispheres ; 18, the 

 optic lobes or corpora quadrigemina. 



