212 THE PROG. 



4. The Appendicular Skeleton. 



The limbs arise about the time of completion of the oper- 

 cular fold, and shortly after the opening of the mouth, i.e. in 

 tadpoles of about 11 or 12 mm. length. 



The hind limbs (Fig. 83, LP) appear as a pair of small 

 rounded buds from the ventral surface of the hinder part of the 

 body wall, at the base of the rectal spout. 



The fore limbs (Fig. 84, LA) are similar buds, which arise 

 about the same time, from the sides of the body wall at its 

 anterior end, opposite the head kidneys. They lie in the dorsal 

 angle between the body wall and the opercular fold, and, being 

 covered by this latter, are not visible from the surface. 



The limbs grow from the somatopleure alone ; each is a solid 

 mass of compact mesoblast, covered by a cap of epidermal cells r 

 which differ in their cubical or columnar shape from the flat- 

 tened cells of the general surface of the body. 



The limbs at first grow slowly. They gradually elongate, 

 become segmented, and then divided distally into fingers and toes 

 (Fig. 85, LA, LP). Up to the time of the metamorphosis the 

 hind limbs are small, while the fore limbs remain concealed 

 within the opercular cavity. During the metamorphosis (Fig. 

 86), both pairs of limbs grow rapidly, more especially the hind 

 limbs. 



The skeleton of the limbs, and of the limb-girdles by which 

 the limbs are attached to the body, does not assume definite 

 form until a short time before the metamorphosis. 



a. The pectoral girdle develops as two half-rings of carti- 

 lage, one 011 each side of the body, which they encircle at the 

 level of the second or third vertebra. The dorsal ends of the 

 half-rings (Fig. 88, ss) lie superficially to the transverse pro- 

 cesses of the vertebras, FT, and are connected with these by 

 muscles and ligaments ; the ventral ends, CO, meet each other 

 in the median plane. 



Each half-ring has on its outer surface, rather below the 

 middle of its length, a cup-shaped depression, the glenoid 

 cavity. GC, with which the head of the humerus, HU, articu- 

 lates. The part of the ring above the glenoid cavity is the 

 scapular portion, the part below it the coracoid portion. 



The scapular portion is divided, shortly before the metamor- 



