THE HEAD KIDNEY AND ^YOLFFIAN BODY. 191 



are in contact with each other, owing to the distension of the 

 body by the mass of food-yolk, except at the upper or dorsal 

 angle. The glomerulns, therefore, appears at this stage to lie 

 in a special cavity, which later on opens into the general body 

 cavity, as the yolk becomes absorbed and the abdominal viscera 

 acquire more definite form. 



At a later stage still, the part of the body cavity in which 

 the glomerulus lies becomes partially boxed in, by fusion of 

 the outer wall of the lung with the peritoneal covering of the 

 head kidney (c/. Fig. 84). This inclosure is only an incomplete 

 one, as the part of the ccelom lodging the glomerulus still opens 

 into the general body cavity, both in front of the root of the lung 

 and behind this. 



The position of the glomerulus, opposite the nephrostomes 

 of the head kidney, and the fact that its development, both 

 progressive and retrogressive, keeps pace with that of the head 

 kidney, point to the existence of some close physiological 

 connection between the two organs, though it is not easy to 

 imagine of what precise nature this connection can be. 



The glomerulus, with the part of the body cavity in which it 

 lies, may be compared to one of the Malpighian bodies of the 

 adult kidney ; the glomerulus itself corresponding to the 

 capillary knot of vessels ; the localised part of the coelom in 

 which it lies corresponding to the capsule of the Malpighian 

 body ; and the nephrostomial tube leading from the coelom 

 answering to the neck of the Malpighian body. 



3. The Wolffian Body. 



The development of the Wolffian body, or kidney, commences 

 in tadpoles of about 10 to 12 mm. length, by the formation of 

 the Wolffian tubules (Fig. 83, KM). These appear as a series 

 of small, paired masses of mesoblast cells, lying along the inner 

 sides of the segmental ducts, between these and the aorta. The 

 Wolffian tubules develop from behind forwards ; the hindmost, 

 and at first the largest pair, being a short distance in front of 

 the cloaca, and the most anterior pair being about three segments 

 behind the head kidneys. They are at first segmentally arranged, 

 one pair corresponding to each pair of muscle segments or 

 myotomes ; but this definite arrangement is early lost in the 

 hinder region. 



