GENERATION AND DEVELOPMENT. 



275 



Similarly, the glosso-pharyngeal divides to enclose the second visceral 

 cleft, its lingual branch being distributed to the second, and its pharyn- 

 geal branch to the third arch. 



rr 



FIG. 431. For description see Fig. 429. 



The vagus, too, sends a branch (pharvngeal) along the third arch, and 

 in fishes it gives off paired branches, which divide to enclose several suc- 

 cessive branchial clefts. 



DEVELOPMENT OF THE EXTREMITIES. 



The extremities are developed in a uniform manner in all vertebrate 

 animals. They appear in the form of leaf -like elevations from the pari- 



FIG. 432. A human embryo of the fourth week; 3} lines in length. 1, 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; 11, that of the lower extremi- 

 ty; 12, the umbilical cord; 15, the eye; 16, the ear; 17, cerebral hemispheres; 18, optic lobes, corpora 

 quadrigemina. (.Mliller.) 



etes of the trunk (see Fig. 432"), at points where more or less of an arch 

 will be produced for them within. The primitive form of the extremity 



