REPR OD UCTION. 483 



stood. It is conceivable that it may arise from the presence of two nuclei 

 within the one ovum. It is more probable, however, that it is due to a 

 mechanical separation of the blastomeres after the first cleavage or later in 

 segmentation. 1 Driesch, 2 Wilson, 3 Zoja, 4 and others have shown that in various 

 invertebrates and the low vertebrate Amphioxus, single blastomeres, isolated 

 from the rest by shaking or other unusual treatment, are capable of develop- 

 ing into small but otherwise normal and complete embryos. Xo reason is 

 obvious why such an occurrence cannot take place in human development, if 

 in any accidental manner within the Fallopian tube the blastomeres become 

 separated. Driesch observed in the sea-urchins and AVilson in Amphioxus 

 that incomplete separation of blastomeres produced two incomplete organisms 

 more or less united together. It is not improbable that even in man cases 

 like the Siamese Twins, and greater monstrosities, may be similarly accounted 

 for. In cases of double pregnancy from a single ovum the two amnions are 

 usually separate, in rare cases a breaking away of their partition wall throwing 

 them into one; the two placentas usually fuse more or less into one, the blood- 

 vessels of the two halves always anastomosing ; and a single decidua reflexa 

 covers both. The two offspring are uniformly of the same sex and their per- 

 sonal resemblance is always close. 



In Veit's statistics of 13,000,000 births in Prussia, triplets occur witli a 

 frequency of 0.012 per cent., or 1 in 7910, and quadruplets 1 in 371,126 births. 

 There are well-authenticated cases of quintuplets. In all of these cases a 

 single ovum rarely, if ever, contributes more than two embryos, and these 

 are characterized, as in the case of twins, by being of similar sex, by pos- 

 sessing a single chorion, and by close personal resemblance. 



The Determination of Sex. — In most, if not all, civilized races more boys 

 are born than girls. This is shown in the following table: 5 



Boys bom to 1000 Girls born (1887-91). 



Italy 1058 England 1036 



Ireland 1055 



German Empire 1052 



Erance 1046 



< lonnecticut 1072 



Rhode Island L049 



Massachusetts 1046 



The proportional birth-rate of the two sexes is usually fairly constant from 

 year to year. This means that constant regulating factors are at work. 

 What determines sex in any one individual is ill understood. The sexual 

 organs in the human embryo are well differentiated at the eighth week of 

 intra-uterine life, hence the sex of the child must be settled previously to this 

 time. It is at present quite impossible to say whether it is settled in the 

 germ-cells previous to their union, in the act of fertilization, or during the 

 early uterine life. Many facts, both observational and experimental, and 



1 Cf. Er. Ahlfeld : Arehiv fur Qynakologie, i\., 1876. 



2 II. Driesch : Zriixrhrifi fiir nissenschaftliche Zoologie, liii , 1892; lv., 1898; MiUheilungen 



mis dir Zonlni/isrltrn Station zn Xi/ip,/, xi., 1893. 



? E. R. Wilson: Journal of Morphology, viii., L893. 



4 R. Zoja : Ar<-liir fiir Entwickelungsmechanik der Organismen, ii., 1895. 



5 Bulletin de I'institut vnternational </< statislique, vii. 



