244 



The Journal of Heredity 



formed. It used to be supjjosed that 

 identical twins resulted when the two 

 halves of the fertilized egg separated 

 after the first division, instead of re- 

 maining in contact as they regularly do. 

 But this idea is now considered crude. 

 Newman has to confess that ' ' the prob- 

 lem of the exact origin of dui)licate 

 human twins is likely to remain un- 

 solved for a long time to come," but this 

 refers only to the details of the process; 

 there is no doubt that they come from 

 a single egg-cell, in which two centers of 

 growth appear very soon after fertiliza- 

 tion. 



ARMADILLO TWINS 



Some species of armadillo ])r()(huv 

 duplicate twins regularly at each Ijirth; 

 the species which lives in Texas jjro- 

 duces quadru])lets by the same j^rocess; 

 while a South American sj^ecies regularly 

 gives birth to eight or even twelve 

 young, all derived from one original egg- 

 cell. The armadillo, therefore, offers 

 excellent material for a study of the 

 process of twinning, and Newinan, who 

 has been one of the principal students 

 of it, devotes much space to it in the 

 book under review. 



It has been found that for several 

 weeks after the egg of the armadillo has 

 been fertilized, it remains quiescent and 

 undergoes little development. It is this 

 retardation, Newman believes, which 

 results in the establishment of four dif- 

 ferent growing j^oints, leading to four 

 separate offspring, in ])lace of the 

 ordinary single growing i:oint \\-hicli 

 would result in a single offspring. But 

 what causes this slowing down? New- 

 man, in 1913, made the rather startling 

 statement that it was due to the presence 

 of a microscopic parasite, ]jrobaVjly 

 a protozoan, in the egg-cell. He now 

 feels less confidence in this belief. The 

 parasite may be the cause of retarded 

 dcvelojjment, or merely one of the con- 

 sequences of it. 



THE FRIiliMAKTIN 



Cattle breeders have long been famil- 

 iar with a i)eculiar result of twinning. 

 When a cow gi\es birth to twin calves (f 



the same sex. they are ordinarily quite 

 normal, but if they are of opposite sexes, 

 the female is sexually imperfect — so 

 much so that until recently it has been 

 supposed that this abnormal twin was 

 really a male. It is called a free- 

 martin, and is always sterile.^ The 

 mystery of the sterile twin was cleared 

 up recently by F. R. Lillie. Twins in 

 cattle, he found, always result from two 

 separate eggs. At a very early period 

 in elevelopmcnt the two embryos lie 

 side by side, partly united, and a con- 

 stant interchange of blood takes place 

 between them. If both are males or 

 both are females, no harm is done; but 

 if one is a male and the other a female, 

 the reproductive system of the female is 

 largely suppressed, and certain male 

 organs even develop in the female. 

 This is unquestionably to be inter- 

 preted as a result of hormone action. 

 The hormones are secretions from the 

 reproductive tissues, which are neces- 

 sary for the develo]5ment of the repro- 

 ducti\'e organs. Probably the hor- 

 mones of the male become active sooner 

 than those of the female, in the em- 

 bryonic period. The interchange oi 

 fetal blood carries male hormones 

 through the female and prevents the 

 development of the normal female 

 reproductive organs. 



Freemartins sometimes occur in sheep, 

 according to W. Bateson, but ordina- 

 rily sheep tw^ins are i)erfcctly normal, 

 even if of opposite sexes. Freemartins 

 are not know'n in other animals. 



TWINS AND SEX-DETKRM I NATION 



The problems of sex are today at- 

 tracting the widest attention, and 

 among these problems that of the 

 mechanism of sex-determination ap- 

 l)ears to have been largely solved. It 

 a])i)ears that in a vast number of 

 animals of all grades of organization, 

 from w'orms to man, sex is determined 

 at the time of fertilization of the egg- 

 cell. In some forms sex is determined in 

 the egg, for there are two distinct tyixs 

 of eggs, male-i)roducing and female- 

 producing. In other cases the e'ggs are 

 all alike and ])r()duce females if allowed 



'About one lime in c'i>»lit, a normal f<ii>.ilr . .ill' is Ix.rn twin witli a niali-. 



