198 THE FROG. 



and function. The tubules of these Malpighian bodies do not 

 join those of the other parts of the kidney, but open at once 

 into the terminal or collecting tubules of the Wolffian body, so 

 that there is practically a sharp separation of the kidney into two 

 portions, excretory and reproductive respectively. 



7. The Fat Bodies. 



The fat bodies, which are conspicuous and characteristic 

 structures in the adult frog, are formed by fatty degeneration of 

 the anterior ends of the genital ridges. In tadpoles of about 

 40 mm. length (Fig. 85) each genital ridge becomes divided by 

 a slight constriction into two parts of about equal length. Of 

 these, the posterior part, OR, forms the smooth genital ridge ; 

 while the anterior part, F, is irregularly notched along its 

 ventral surface, and is already commencing to undergo fatty 

 degeneration. In the later stages (Fig. 86, F) the fat bodies 

 increase greatly in size, and are produced at their margins into 

 the characteristic finger-like lobes of the adult. 



Histologically, the changes by which the fat body is formed 

 are due to the accumulation of fat within cells, which are at 

 'first indistinguishable from those of the hinder, or reproductive, 

 part of the genital ridge. 



DEVELOPMENT OF THE SKELETON AND 

 THE TEETH. 



1. The Vertebral Column. 



The earliest skeletal structure, and for a time the only one, 

 is the notochord, the development of which, from the mid-dorsal 

 wall of the mesenteron, has already been described (p. 109). 



The notochord (Fig. 61, CH) is a cylindrical rod, extending 

 from the blastopore to the pituitary body ; and growing back- 

 wards along the tail as this is formed (Fig. 69, CH). It consists 

 of vacuolated cells, filled with fluid : the cells are all alike, and 

 their nuclei, which at first are conspicuous, disappear in the 

 later stages. Around the notochord a delicate structureless 

 elastic sheath is formed at an early period ; this is really 

 double, consisting of a thicker inner, and an outer thinner layer ; 

 both layers are homogeneous, and cuticular in nature. 



About the time of appearance of the hind limbs as buds, a 



