REPRODUCTION. 921 



Observations of discharged Graafian follicles in cases of multiple concep- 

 tions show that twins may arise either from separate eggs or from a single egg. 

 The presence at birth of a double chorion is commonly regarded as diagnostic 

 of the former origin, that of a single chorion of the latter. In the former 

 case the two ova may come from a single Graafian follicle, or from two folli- 

 cles situated within one ovary, or from both ovaries, direct observation of the 

 ovaries themselves being required to determine the origin in any particular 

 case. The two ova are discharged and fertilized probably at approximately 

 the same time. There are two distinct amnions. The two placentas may be 

 either fused into one or wholly separated from each other, and accordingly the 

 deeidua reflexa may be single or double. The two offspring may be of sep- 

 arate sexes, and do not necessarily closely resemble each other. In cases 

 where the two embryos come from a single ovum their origin is little under- 

 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. No 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 Wilson in Amphioxus 

 incomplete separation of blastomeres to produce 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 deeidua 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 with 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 : 6 



1 Of. Fr. Ahlfeld : Archiv fur Gynakologie, ix,, 1876. 



2 H. Driesch: Zeitschrift fur wissenschaftliche Zoologie, liii., 1892; lv., 1893; Mittheilungen 

 aus der Zoologischen Station zu Neapel, xi., 1893. 



3 E. B. Wilson : Journal of Morphology, viii., 1893. 



4 R. Zoja : Archiv fur Entwickdungsmechanik der Organismen, ii., 1895. 



5 Bulletin de I'institut international de statistique, vii. 



