Kellicott, A Contribution to the Theory of Growth. rgg 



weight. In Mustehis the relation between weight and length is very 

 simple, but length gives no better indication of the real growth 

 relations and processes. The actual relation here is such that 

 the addition of equal increments in weight is accompanied by 

 the addition of constantly diminishing increments in length (dia- 

 gram). While the fish after birth may increase in weight ioo 

 times, they increase in length meanwhile only 4 times. 



What then do we~Tneasure when we attempt to measure 

 growth by observing changes in total weight? Obviously the 

 complex net resultant of the separately and variously growing 

 parts. But what is the significance of the change in total weight? 

 Frequently this gives us the rate of growth of some one or two 

 preponderating parts or tissues. In the vertebrates, for example, 

 the muscles and connective tissues make up approximately 75 % ♦ 

 of the total weight of the organism. It is therefore chiefly the 

 rate of growth of these tissues that gives character to the growth- 

 rate of the whole organism. And yet the growth of these parts 

 may be of lesser importance to the organism than that of other 

 parts such as the brain or heart or fins. 



Apparently we have here morphological evidence bearing 

 upon the method of growth and its regulation in the animal. 

 In a few cases among plants it is known that growth of certain 

 parts, e. g., roots, is inhibited by the presence of specific chemical 

 substances, in this case resulting from the growth process itself. 

 On the other hand, among the higher vertebrates the normal 

 appearance and growth of certain occasional organs or tissues, 

 such as the mammary glands and decidua, have been shown to 

 be due to the effects of specific internal secretions or "hormones" 

 produced by some part of the organism. The growth of many 

 if not most of the so-called secondary sexual characters undoubt- 

 edly results from the presence of internal secretions of the re- 

 productive glands. Students of Mendelism believe that the length 

 of the hair is controlled by a specific factor which inhibits the 

 growth of the hair when it has reached a certain length. The 

 specific regulatory effect of the internal secretions of the thyroid 

 and pituitary bodies upon the growth of the brain, subcutaneous 

 tissues, and bones, are now well known. In cases of under-feed- 

 ing all the tissues of the organism are not similarly affected 

 but certain tissues or organ-systems grow almost normally while 

 the growth of others may be inhibited to a very marked degree. 

 Many believe that the growth of some forms of tumors and similar 

 cell-proliferations is due to specific internal secretions, specific 

 for a given kind of tissue. Such secretions may be 

 produced by some part of the organism itself or by some para- 

 sitic organism. Many of the best illustrations of the latter re- 

 lation are afforded by the gall-structures of plants, in which specific 

 tissues may be formed, and among animals by the equivalent cysts. 



