TWINNING IN DASYPUS NOVEMCINCTUS 67 



definitive villi. Later, when the four embryos form 

 an attachment at their posterior part With the Trager 

 ring, a co-operation between embryonic endoderm 

 and the Trager epithelium takes place, resulting in the 

 formation of hollow vascular vilU that invade deeply 

 the maternal mucosa. Four separate patches of villi 

 appear corresponding to the point where each embryo 

 has developed its own nutritive attachment with the 

 mother. With the rapid peripheral extension of those 

 villous patches there is an apparent fusion of the four 

 placentae into a scalloped placental ring which appears 

 to be continuous, but is not, for there is no admixture 

 of blood between adjacent fetuses. This has been 

 fully demonstrated by injections. This "compound 

 zonary placenta," as it has been called, remains as an 

 apparently complete ring about the equator of the 

 vesicle until rather late stages of development. Then 

 there follows a separation of the ring into two almost 

 separate discoid placentae, each of which receives the 

 umbilical cords of a pair of embryos. If one examine 

 a gravid uterus during the last month of pregnancy, he 

 will readily note that in the median dorsal and median 

 ventral regions there is an area almost devoid of pla- 

 cental tissue. If the uterus be cut open along this clear 

 line on the ventral side, the cut will fall between the 

 placental disks of the two halves of the uterus, and the 

 orientation of pairs will usually be preserved. 



