294 



The Journal of Heredity 



is intimately concerned with quantita- 

 tive development. 



Clinical observations on man and 

 extirpation experiments on animals 

 have established the fact that a defi- 

 cient thyroid secretion results in a 

 marked retardation of body growth. 

 It should be noted that present evi- 

 dence supports the belief that the 

 retardation accompanying thyroid de- 

 ficiency is a consequence of the low- 

 ered plane of general metabolism. The 

 earher idea that the thyroid gland 

 produces a secretion specifically stim- 

 ulative of growth has yet to he sub- 

 stantiated. 



The studies of Swingle and others 

 have demonstrated the significant role 

 of the thyroid in differential develop- 

 ment of amphibia in relation to meta- 

 morphosis. No systematic quantita- 

 tive studies of the part played by the 

 gland in differential development of 

 mammals had been made up to some- 

 what over three years ago when I 

 began experiments, the detailed results 

 of which are being published in a 

 series of papers in other journals. The 

 plan of the investigation and the pro- 

 cedures employed are to be found m 

 these communications. This paper 

 deals with two of the striking evi- 

 dences of the dependence of normal 

 differential development on thyroid 

 function and certain implications to be 

 derived therefrom. The observations 

 were made on albino rats of both 

 sexes, adequately controlled as to litter, 

 sex, age, diet, and environment, during 

 the growth period from 100 to 150 

 days of age. The test, or thyroidless 

 rats, were deprived of the thyroid 

 a])])aratus (thyroid plus parathyroids) 

 when 100 days of age and allowed to 

 grow until 150 days old when they 

 were killed and the appropriate meas- 

 urements made. The values so ob- 

 tained were compared with those 

 obtained from the unoperated controls. 

 It was found, as others have found, 

 that the loss of the thyroid secretion 

 caused a marked retardation in the 

 growth in weight of the body as a 



whole. That is to say, total quanti- 

 tative development was retarded. In 

 addition is was found that the retard- 

 ation was not a simple proportionate 

 retardation in which the organism 

 at the terminal stage showed the same 

 relative composition as the control, 

 l^he various organs differed in the de- 

 gree of their response to the thyroid 

 deficiency. The various organs did 

 not show proportionate degrees of re- 

 tardation of the rate and extent of de- 

 velopment. Disproportionate variation 

 in the development of different organs 

 was caused. A "type" of organism re- 

 sulted which was different from the 

 normal for the group from which the 

 test animals came. When the internal 

 environment was altered by the re- 

 moval of the thyroid apparatus a 

 differentiation of development ob- 

 tained. 



This effect is quite marked in the 

 growth of the central nervous system, 

 as represented by the brain and spinal 

 cord, taken as separate organs. 



In order that the effect and its ex- 

 tent may be made clear, there is given 

 in Figure 1 the growth of the brain 

 and spinal cord of the thyroidless rats 

 in terms of the growth of these struc- 

 tures in the controls. The solid black 

 column represents the growth in con- 

 trols as 100 per cent. The heights 

 of the other coknnns represent the 

 degree of development of the brain 

 and spinal cord of the male and female 

 thyroidless rats, or the degree to which 

 these organs failed to reach the normal 

 level. 



From the chart it is at once evident 

 that the removal of the thyroid func- 

 tion does not have the same quantita- 

 tive effect upon the development, as 

 measured by the weight, of the brain 

 as it does upon the spinal cord. The 

 growth of the former is much more 

 markedly retarded than is the growth 

 of the latter. The lack of thyroid 

 secretion caused a disproportionate 

 variation in the differential develop- 

 ment of these two parts of the nervous 

 system of one and the same individual. 



