4/6 DEVELOPMENT OF ELASMOBRANCH FISHES. 



by Professor Semper, who figures it at an early stage of develop- 

 ment, and implies that it arises in the 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 n, fig. 9*7, sti). 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 for a considerable distance in the region of the small 

 intestine. It retains up to stage O its original extension, but 

 the anterior part becomes quite definite though still of a smaller 

 calibre than the posterior. In one of my examples of stage O 

 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 succeeding stages up 

 to Q (PL 1 8, 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) of these two 



