Growth of Brain and A^iscera in Dogfish 331 



though the concavity decreases considerably toward the upper end. The 

 curve most nearly similar to this is that of the brain (Plate 1) in which 

 the concavity is well marked, but does not extend over the entire curve, 

 the upper end being straight though at an angle with the base. Fol- 

 lowing this we have the curve of the pancreas (Plate 4), which is 

 considerably less concave and in which the entire upper part has become 

 straight. This tendency to straighten is continued in the curve of the 

 rectal gland (Plate 3) where only the very beginning shows any con- 

 cavity and that very slight. Finally we reach the condition found in 

 the heart (Plate 2) where the line becomes straight almost from its 

 very beginning. The curve given by the liver weights (Plate 6) is 

 more like this last than any other, but is not simple in character for 

 reasons to be mentioned presently. This tendency for the concave 

 curves to straighten is carried to an extreme in the gonad curves (Plate 

 7) which actually become convex to the base; this is most pronounced 

 in the curve of the testes. 



Of course the alteration here of a concavely curved line into a straight 

 line indicates that an organ passes from a condition in which it grows 

 by the addition of constantly diminishing increments, to one of growth by 

 the addition of successively equal increments, the total weight meanwhile 

 increasing in either case by the addition of equal amounts. The spleen, 

 for example, throughout its growth increases by the addition of constantly 

 decreasing increments, while the heart almost from the first grows by 

 the addition of equal increments. The other organs show the various 

 intermediate conditions described. The obliquity of this line to the base 

 is determined by the size of the increment added, just as the amount 

 of the curvature is determined by the rate at which the increments are 

 decreasing. The rectal gland is the only one of the viscera except the 

 gonads which gives a curve becoming convex to the base, and this occurs 

 only at its extremity, this means that this organ increases, among the 

 largest females, by the addition of increments of increasing size. 



We may conclude then that these curves though all of the same gen- 

 eral type, are individually distinct so that the rates of increase in weight 

 of these organs are not determined by precisely the same factors or, 

 at any rate, by factors operating with equal intensity at corresponding 

 periods throughout this series of organs. This is shown also by the 

 curves showing the relative weights of these organs which seem more 

 instructive and which aid in the possible interpretation of these curves 

 just described. 



