422 



The Journal of Heredity 



the mother. Careful observations along 

 these lines are much to be desired. It 

 has been shown that dwarfism in its 

 \arious forms tends to be congenital, as 

 shown in Rischbieth and Harrington 

 "Treasury- of Inheritance." 



CONCLUSIONS 



The last quarter centur\- has wit- 

 nessed a most brilliant series of dis- 

 coveries in c>tologA' and heredity. We 

 talk a jargon in which are heard such 

 terms as ISIendelism, allelomorphs, 

 genes, sex-determination, sex-linkage, 

 crossing-over, etc. We have built up 

 an edifice of theory^ that is amazing 

 and a bit bewildering to one who can 

 not lay claim to being a specialist. 

 Out of it comes one insistent impres- 

 sion, namely, that our fate is sealed at 

 the time of union of egg and sperm — 

 that our future is decreed in the 

 chromosomes. It is no doubt true that 

 the nuclei of the germ cells carry 

 definite tendencies that interact upon 

 one another to shape and direct 

 development. 



On the other hand it is equalh- true 

 that other factors are at work shaping 

 the course of development. Most 

 prominent among these are the endo- 

 crine glands. The writer freely admits 

 that the character and functional 

 activity of these glands may be in large 

 part determined by hereditary tenden- 

 cies, yet disease and accident play at 

 times a very important role. It has 

 been clearly shown tiiat development 

 toward maturity in the tadpole of the 

 leopard frog will proceed only to a 

 certain point in the absence of the 

 thyroid gland or of the anterior 

 lobe of the pituitary gland. Ha\ing 

 reachefl this i)oint the process ceases. 

 In the common toad this process of 

 growth toward maturity continues to a 

 somewhat later stage than in the frog 

 and there ceases just as abruptly. 

 Beyond these points there may be 

 very slight further progress but it is 

 practically negligible. It is interest- 

 ing to note tiiat : 



1. The stage reaclud is a definite 

 one peculiar to the sjKcii's. I)e\elop- 

 ment up to this jioint is f|ui(c normal, 

 leaving out of accoiml a slight retarda- 

 tion in si/e growth in the absence of 

 the pituitary gland. 



2. This stage is reached equally in 

 the absence of the thyroid gland, of 

 the anterior lobe of the pituitary' gland 

 or of both, and further development is 

 possible only in the case of 



(a) transplantation of the missing 

 gland or glands or 



(b) feeding with thyroid gland ma- 

 terial or 



(c) feeding with large quantities |of 

 iodine. 



Now the practical value of these 

 discoveries lies in the fact that they 

 show us ways not only of correcting 

 certain types of abnormal size growth 

 and development; but give us hope 

 that we may some day modify what we 

 consider to be normal growth and 

 development. This may at some fu- 

 ture time be carried out in a thor- 

 oughly practical way. While we can 

 readily cause half grown tadpoles to 

 quickly develop into little weazened 

 frogs by feeding them thyroid prepara- 

 tions or iodine, it still remains to be 

 seen how far we may stimulate them to 

 size growth beyond the normal limits 

 of the species by any process of admin- 

 istration of the anterior lobe of the 

 pituitary gland. Uhlenhuth has shown 

 that feeding pituitary preparations to 

 Ambystoma causes giant growth in 

 size. This fits in with our theories 

 regarding the causation of gigantism. 



We may be permitted the hope that 

 through our knowledge of these glands 

 we may be able to largely control the 

 rate and extent of developmental and 

 size-growth processes. May the writer 

 be permitted in closing to again express 

 his conviction that the amj-jhibians 

 afford the best opportunities for the 

 accurate investigation of these ques- 

 tions. They should play a role in the 

 study of these promisingly practical 

 problems comparable to the part 

 played by the insects in the field of 

 cytology and experimental breeding. 

 Indeed they are far closer home be- 

 cause they are vertebrates like our- 

 selves — the forms nearest of kin in 

 which these glands can be reached by 

 operation at their inception, and in 

 which feeding experiments may be car- 

 ried on at an earh- stage of develop- 

 ment. 



