246 DEVELOPMENT OF INTERRENAL BODY. 



mesoblast and in connection with the suprarenal body. It 

 appears at stage K as a rod-like aggregate of mesoblast cells, 

 rather more closely packed than their neighbours, between the 

 two kidneys near their hinder ends (Plate XI. fig. 9tx, sii). The 

 posterior and best marked part of it does not extend further 

 forwards than the front end of the large intestine, and 

 reaches backwards nearly as far as the hinder end of the 

 kidneys. This part of the body lies between the caudal vein 

 and dorsal aorta. 



At about the point where the unpaired caudal vein 

 divides into the two cardinals, the interrenal body becomes 

 less well marked off from the surrounding tissue, though it 

 may be traced forward from a considerable distance in the 

 region of the small intestine. It retains up to stage Q its 

 original extension, but the anterior part becomes quite de- 

 finite though still of a smaller calibre than the posterior. 

 In one of my examples of stage the two divisions were 

 separated by a small interval, and not as in other cases 

 continuous. I have not determined whether this was an 

 accidental peculiarity or a general feature. I have never 

 seen any signs of the interrenal body becoming continuous 

 with the suprarenal bodies, though, as in the adult, the two 

 bodies overlap for a considerable distance. 



The histology of the interrenal body in the embryonic periods 

 is very simple. At first it is formed of cells differing from those 

 around in being, more circular and more closely packed. By 

 stage L its cells have acquired a character of their own. They 

 are still spherical or oval, but have more protoplasm than 

 before, and their nucleus becomes very granular. At the same 

 time the whole body becomes invested by a tunic of spindle- 

 shaped mesoblast cells. By stage O it begins to be divided 

 into a number of separate areas or lobes by septa formed 

 of nucleated fibres. These become more distinct in the suc- 

 ceeding stages up to Q (PL xvii. fig. 7), and in them a 

 fair number of capillaries are formed. 



From the above description it is clear that embryology 

 lends no more countenance than does anatomy to the view 

 that the interrenal bodies belong to the same system as the 

 suprarenal, and it becomes a question with which (if of either) 



