350 H. H. NEWMAN 



stopped development in some of the more susceptible trees, and 

 those flower buds that were in the most sensitive condition were 

 most severely inhibited. When seasonable weather returned 

 the flower buds capable of recovery resumed growth, but the 

 most apical region had become so seriously injured that it was 

 incapable of continuing. In consequence, symmetrical regions 

 of the growing tip near the original apical point became the 

 new apical points of the twin fruits. 



The causes of human twinning 



Davenport has recently published data that tend to show that 

 monozygotic twinning is inherited strongly through the male 

 parent. If this finding is valid, how could this fact be shown 

 to accord with the general theory of twinning herewith ex- 

 pounded? If in human beings twinning be a result of temporary 

 retardation followed by recovery, how could the sperm be re- 

 sponsible for retardation? It is evidently true that in human 

 beings, as in other animals, there are varying degrees of compati- 

 bility between the eggs of some females and the sperms of some 

 males. Doubtless the eggs of some females are totally incapable 

 of fertilization by the sperms of some males, while quite fertile 

 to the sperm of others. Doubtless also there are many border- 

 line cases that involve relative incompatibility and consequent 

 disharmony and retardation. If retardation be sufficiently 

 severe, physiological isolation might occur at a relatively early 

 period, which would likely result in completely separate twins; 

 but if physiological isolation occurred relatively later, there would 

 be a less complete separation of the twin bodies, and the resultant 

 conjoined twins, cases of dicephaly, spina bifida, and others 

 types of teratological duplication common in human fetuses. 

 Thus twinning might be inherited through the male line owing- 

 to some peculiarity of sperm in a given race that has a retarding 

 effect upon the egg. 



