A CRUCIAL TEST OP GENETIC CONSTITUTION AS AGAINST 



ENDOCRINE DISTORTION IN DETERMINING THE TYPE 



OP STRUCTURAL FORMATION: THE " SCREW-TAIL" 



IN BULLDOG HYBRIDS 



Many conditions have already been discussed which indicate 

 that the genetic composition of the individual may bring 

 about localized growth distortions in otherwise normal bodies. 

 One of the most completely investigated cases of localized 

 distortion is recorded in the earlier section on achondroplasia 

 in the skeleton of the extremities. This condition is clearly 

 inherited in a very smple fashion, but when considered alone 

 there is, nevertheless, the possibility that an early and tem- 

 porary endocrine modification may have been the primarily 

 inherited character acting to initiate the chondrodystrophy 

 in the extremity skeleton without similarly affecting the axial 

 skeleton which at that particular period was not in a suscep- 

 tible stage. The critically susceptible stages in the develop- 

 ment of the appendicular and axial portions of the skeleton 

 probably occur at different developmental moments. 



The reverse condition of localized achondroplasic modifica- 

 tion of the head and tail ends of the axial skeleton in a body 

 with normal growth of the extremities, therefore, lends itself 

 to a similar possible explanation. Also the further fact, 

 brought out in a previous section, that the homozygous state 

 for extremity achondroplasia brings about a more severe 

 expression of this condition than does the heterozygous is 

 still open to explanation on the basis of possible differences 

 in degrees of endocrine modification resulting from mixed 

 and pure allelic pairs. Even the different growth reactions 

 shown by the upper and lower jaws in the same head may 

 be due to momentary endocrine disturbances to which the 

 growth of one jaw is more susceptible than the other. Such 

 temporary endocrine disturbances are known to modify growth 

 when acting during particularly susceptible periods. 



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