4i6 



The Journal of Heredity 



development of the former is con- 

 trolled by the thyroid ^dand while the 

 gonads develop independentK' of it. 

 We should not at this time lay too 

 great emphasis upon this point, but it is 

 at least suggestive. If substantiated 

 by further investigations, such a radical 

 difference between soma and germ cells 

 would be most significant in \iew of the 

 theories of heredity that are based upon 

 the assumption that the germ-cells are 

 not influenced by changes in the soma. 

 It has been long known that iodine 

 is the most characteristic chemical 

 constituent of the thyroid secretion; 

 it is therefore interesting to find that 

 by feeding iodine, Swingle produced 

 the same effects of metamorphosis in 

 tadpoles that are produced by feeding 

 thyroid preparations. We are thus 

 led to the conclusion that the iodine 

 accumulated by the thyroid gland is 

 the active agent in the acceleration 

 development. Kendall's interesting 

 work in the analysis of the thyroid 

 secretion should be taken into account. 

 He has isolated the substance thyroxin, 

 the constituent richest in iodine and 

 considers this compound to be the ac- 

 tive agent involved. It remains to be 

 seen whether the simple iodine element 

 alone can accelerate development in 

 forms other than the amphibians. 

 There is no question about its potency 

 in that group. .Swingle was even able 

 to show that tadpoles deprived of the 

 thyroid gland can be made to trans- 

 form when fed iodine. 



THYROID FEEDING CAUSES SHRINKAGE 



While the thyroid gland has a 

 marked influence upon the process of 

 differentiation, it appears to regulate 

 size growth secondarily through its 

 influence upon the attainment of ma- 

 turity. Feeding or imj)lantation of the 

 thyroid gland lirings about a distinct 

 shrinkage of tadpoles so treated. This 

 has been by some described as a 

 "burning up" process due to greatly 

 accelerated metabolism. This might 

 prove true but we should be cautious 

 and demand proof upon this point. It 

 is well known that the tissues of 

 amphibians, especially in the larval 

 stage contain an enormous percentage 



of water. Only careful weighing of 

 dried specimens can serve to determine 

 whether th>roid or iodine feeding does 

 Or does not cause an actual wasting of 

 the essential substance. Students in 

 this laborator>' are now working upon 

 this question and upon the relation of 

 the thyroid glands to the fat bodies — 

 a nearh- allied problem. Certain it is 

 that thyroid feeding causes shrinkage 

 rather than size growth. 



THE HYPOPHYSIS 



The hypophysis, often called the 

 pituitary gland, is really made up of 

 four parts physiologically quite differ- 

 ent from one another, at least this is 

 true of the three portions whose func- 

 tions we are beginning to understand. 

 Three of the four portions arise from a 

 cord of cells that grows inward from 

 the surface layer of the body and comes 

 to lie under the brain. It develops 

 into anterior lobe, intermediate lobe 

 and lateral lobes (pars tuberalis). A 

 fourth portion develops from the tip 

 of the infundibulum of the brain with 

 which the hypophysis comes in contact. 

 These portions are shown in the figure. 

 It is possible to remove the entire cord 

 of cells from which all but the posterior 

 lobe develop. The latter develops in 

 the absence of these portions; but 

 Smith claims that it does not attain 

 normal growth. 



With the removal of the anlage of 

 these lobes (anterior, intermediate, and 

 pars tuberalis portions) certain striking 

 effects are produced, as follows: (1) 

 Prevention of metamorphosis at the 

 same stage attained in the absence of 

 the thyroid gland. (2) Retardation of 

 growth in size. (3) A most striking color 

 change from the normal black to white, 

 due to the contraction of black pigment 

 cells and an expansion of certain silvery 

 white cells, the xantholeucophores. (4) 

 Apparently heightened mortality evi- 

 dent only several weeks after the 

 operation. This was first accomplished 

 by Adler by rather imperfect technique. 

 Later Allen and Smith independently 

 devised a much better operation for 

 removal. The writer has made a par- 

 tial analysis of these functions by im- 

 planting separately the anterior lobe, 



