DEPARTMENT OF EMBRYOLOGY. 85 



mistaken for one of fetal ascites. In addition to the localized anomalies in 

 the region of the hindgut, it is significant to note that the fetus showed definite 

 evidences of death at about the fourteenth week and prolonged intrauterine 

 retention. In view of the preceding paragraphs, this may safely be regarded 

 as an example of defective germ-plasm. 



Formation op Monochorionic Twins. 

 It is only in the past few years that we have begun to emerge from the realm 

 of pure conjecture as to the mechanism of origin of identical twins, and this is 

 because of the scarcity of early material. Fortunately, Dr. G. W. Corner, 

 in examining the uterine contents of a large number of pregnant swine, was 

 able to find three important stages in this process, and thus part of our 

 hypothesis is replaced by direct observation. From his specimen it is clear 

 that such twins arise within a single blastocyst, although it still remains 

 possible that monochorionic twinning may result from the fusion of separate 

 blastocysts with obliteration of the intervening trophoblastic wall. He 

 further shows that twinning in the pig may occur before the formation of the 

 amnion, and in this respect it is quite different from the armadillo, the form 

 on which our most complete descriptions were based. Because of the late 

 formation of the amnion in that order, the possibility of monochorionic 

 twinning in ungulates has been regarded by some writers as impossible. Dr. 

 Corner includes in his study a clear account of the development and arrange- 

 ment of the amniotic and allantoic membranes. It may be added that, as I 

 write these lines, I have before me a still earlier stage that completely sub- 

 stantiates and amplifies Dr. Corner's observations, and in my next report I 

 shall make occasion to refer to this subject again. 



