Growth of Brain and Viscera in Dogfish 335 



digestive organs and to agree more nearly with the muscular and skeletal 

 tissues with respect to its comparative rate of growth in that it does 

 not show on the whole a pronounced relative decrease in weight. The 

 first part of the liver curve (Plate 6) is almost precisely like those of the 

 other organs, but the usual steady decline following the maximal rela- 

 tive size is replaced by only a brief period of decline and then the liver 

 weight rises again to a fairly high level, tending to fall again only among 

 the very largest specimens. It seems probable that this peculiarity is 

 the result, at least in large part, of the storing of fat in this organ. Most 

 higher vertebrates and some lower forms, under good nutritive conditions, 

 lay up fat not only within many tissues but quite usually within or around 

 some of the organs contained in the body cavity, often in large masses. 

 In the dogfish one notices at once the complete absence of fat tissue in 

 this region. The liver, however, particularly in the larger specimens, 

 contains largo quantities of fat or oil, as the manufacturers of cod-liver 

 oil could testify, and everyone who has used these animals in the labora- 

 tory can not have failed to observe this annoying fact. 



In order to fix this point Dr. C.' L. Alsberg, of the Harvard Medical 

 School, has very kindly made, at the Bureau of Fisheries Laboratory, 

 a series of determinations of the amount of fat in livers taken from dog- 

 fish of different sizes. His results show clearly, first, that the amount of 

 fat does increase rapidly with the size of the fish or liver to an extremely 

 high percentage in tlie larger fish; and, second, that livers above or 

 below the average weight in fish of a given size have relatively higher 

 or lower percentages of fat respectively. 



In the dogfish, therefore, as in the teleost (Fulton, '06) the liver 

 must be regarded not as a simple glandular organ, but, in the older 

 individuals, largely as a fat reservoir. This fact offers an interpreta- 

 tion of the form of its growth curves. Only in the younger fish is the 

 liver chiefly a glandular structure, and in such fish we find the growth 

 curve typical for such parts. In animals of 600 to 800 grams we find 

 the accumulation of fat masking the actual condition of growth of the 

 liver tissue proper, so that apparently but not really this organ seems to 

 afford an exception to the rate of growth typical of the viscera. If we 

 could subtract the amount of stored fat from the total weight we should 

 have left a series of weights representing actual liver substance, which 

 would give a curve probably not unlike those already described for the 

 other viscera. The extreme variability of the liver may also result largely 

 from this same fact, since the amount of fat stored up would depend 



