The Development of the Frog 83 



lying along the inner side of the segmental duct, between 

 this and the aorta (Fig. 24, KM}. They develop from 

 behind forwards, the hindermost pair being a short dis- 

 tance in front of the cloaca, and the most anterior ones 

 about three segments behind the head kidney. 



" These solid masses soon become elongated into 

 twisted rods, which then become tubular, and growing 

 towards the segmental duct meet and open into it. At 

 their opposite ends these Wolffian tubules, as they are 

 termed, dilate into bulb-like expansions, which become 

 doubled up by ingrowths of little blood vessels, derived 

 from the dorsal aorta, and so form Malpighian bodies. 

 From the necks of the Malpighian bodies, short solid rods 

 of cells grow towards the peritoneal epithelium and fuse 

 with it. These rods soon become hollow, and open into 

 the body-cavity by ciliated funnel-shaped mouths or 

 nephrostomes: their opposite ends break away from the 

 Wolffian tubules and open directly into the renal veins 

 on the ventral surface of the kidney. The Wolffian 

 tubules rapidly increase in number; they also branch 

 freely, and so give rise to a complicated system of glandu- 

 lar tubules, which, when bound together by blood vessels 

 and connective tissue, form the Wolffian body or kidney 

 of the frog. The nephrostomes persist; and in the adult 

 frog as many as 200 or more are present on the ventral 

 surface of the kidney, as minute funnel-like ciliated 

 openings, leading by short tubes into the renal veins. 



4. The Wolffian and Miillerian Ducts 



" So far we have only described one duct on each 

 side, the segmental duct, which acts as the excretory 

 duct first of the head kidney, and then of the Wolffian 



