Growth of Brain and Viscera in Dogfish 351 



size of the valves of the conus from which the walls of the conus had 

 apparently grown away, thus rendering them incompetent. He found 

 these valves relatively larger in a smaler specimen and in a brief letter 

 to "Nature" suggested that such "normally unequal growth" might be 

 here and elsewhere a cause of natural death. From the facts we have 

 given here it seems that we must expand this idea of "normally unequal 

 growth" to include the general and normal relation, among the lower 

 forms at least, between the locomotor, supporting and protecting tissues 

 on the one hand and on the other the controlling, circulatory and metab- 

 olizing organs, either singly or collectively. 



From this point of view the condition of determinate growth which 

 we find in higher vertebrates is secondary and is derived from that of 

 indeterminate growth as an adaptation upon the part of the organism, 

 such that the muscles and supporting tissues cease their growth at such 

 a point that the brain and viscera remain competent to maintain a 

 physiological balance. We have seen that in the dogfish the brain and vis- 

 cera decrease rapidly in relative size after an early maximum until about 

 the time of sexual maturity and after that the decrease is much slower. 

 If only the muscles and connective tissues could be made to stop their 

 growth at that time (i. e., become determinate), the animal apparently 

 might continue to live for a comparatively long time after maturity. 

 Apparently this is just what happens in the higher forms. 



The recent work of the physiologists upon growth of certain tissues 

 or organs suggests a mechanism through which this normal growth 

 of the tissue or organ might be controlled and which might account 

 for this comparatively more rapid cessation of growth in the determi- 

 nately growing forms. They have demonstrated that the growth of occa- 

 sional organs or tissues often results from the presence of internal 

 secretions or of hormones. For example we might mention the effect 

 of foetus extract upon the growth of the mammary glands, of ovarian 

 secretion upon the growth of the placenta, and so forth. And also in 

 pathological growths internal secretions are known to be involved, for 

 example, the effect of the thyroid secretion upon growth of the brain or 

 connective tissues and of pituitary secretion upon the growth of bones. 

 Probably the best example of this effect of internal secretion upon the 

 growth of pans is given by the whole group of secondary sexual characters 

 which result fi'om secretions of the gonads. These secretions seem to 

 affect growth in either a positive or a negative way — either by their 

 presence or by their withdrawal after having been present for some time. 



