400 H. H. Newman and J. T. Patterson. 



case III and IV exhibit almost precisely the same atypical condi- 

 tion, a short interruption in the first band a little to the left of the 

 median line; II has an interruption in the same band, involving 

 considerably more than half of the total length of the band, while 

 I, although appearing to be perfectly normal, seems to have carried 

 the tendency toward the suppression of a band to the extreme in 

 that the whole band is lacking. In a fourth case one of the four 

 embryos shows a short fusion between the first two rows on the 

 left hand side, while the other three are perfectly normal. 



Three out of four cases, then, furnish strong evidence of pairing, 

 while the fourth case, which is after all atypical only to a mini- 

 mum extent, affords an exception, whose weight can scarcely 

 be sufficient to discredit the evidence of the other cases. 



Although the pairing of embryos is not always perfectly obvi- 

 ous the cumulative evidence in favor of its general occurrence 

 is convincing and must have some fundamental significance, an 

 understanding of which is undoubtedly closely bound up with the 

 early developmental mechanics as we shall attempt to show. 



It has occurred to us that the division of the four-scalloped 

 placental band into right and left lateral discs migh t be dependent 

 upon the fact that the blood supply of the uterus comes from the 

 paired ovarian blood vessels that enter the uterus laterally. It 

 is true that the paired embryos, with very few exceptions are 

 located on the same side of the uterus, but that the pairing is in 

 any way caasally related to the fact of their location near the en- 

 trance of a single maternal blood vessel is highly improbable, 

 because the maternal blood does not reach the embryos. 



It has also been suggested that the close resemblance between 

 the individuals of a pair might be due to admixture of foetal blood, 

 but we have demonstrated by the use of colored injections that 

 the placental area of each embryo is sharply circumscribed and 

 that no blood passes from one embryo to another. A common 

 blood environment then cannot be held accountable for the near 

 approach to identity seen in the pairs. Moreover it has been 

 shown that long before there was any sign of the definitive placen- 

 tation, and hence before there was any circulation of blood, pair- 

 ing of embryos was evident in the relationship of the amriiotic 



