12 THE BIOLOGY OF TWINS 



which there are two separate eggs, either from the same or from 

 opposite oviducts, and implanted at some little distance from one 

 another. In one case investigated by Von KoUiker, the two 

 deciduae were distinct but partially adherent over the surfaces 

 in mutual contact, and in another the contact surfaces had fused 

 into a single wall into which, from the two opposite sides, the 

 chorionic viUi of the two embryos had grown. In addition to 

 this, one of the placentae was of the type known as placenta 

 marginata, caused by a fold of the decidua. (This is evidently 

 a normal multiple birth, a condition hard to accomplish in a 

 uterus of the shape found in human beings, and often attended 

 by such phenomena as adhesions, fusions, and foldings, all 

 indicative of crowding and nothing else.^-Wilder.) 



Case II. — Two separate blastodermic vesicles inclosed in a 

 single decidua. Placentae fused with one another but with two 

 separate sets of umbilical vessels. Two chorions fused at the 

 point of contact. This case is more frequent than (I) but appar- 

 ently results from the same general cause, i.e., two separate eggs, 

 which are, however, implanted nearer together. This w^ould 

 seem more likely to happen if both eggs came from the same side. 

 (The conditions are seen to be similar to those of (I), the greater 

 degree of fusion being well accounted for by the approximation 

 of the two eggs to one another. — Wilder.) 



Case III. — Two amnions and two umbilical cords with a 

 single placenta in the middle of which the two cords meet and 

 ujKjn which the umbilical vessels closely anastomose. These are 

 inclosed in a single chorion and covered with a single decidua 

 reflexa. This case is said by Hyrtl to be more frequent than 

 (I) and (II), but not as frequent, according to Spath. The twins 

 are always of the same sex. Schultze says that the explanation 

 of this singular condition is zweijelhajt and gives the following 

 possible explanations: (i) At first two chorions, as in (II), the 

 contact wall between which becomes absorbed later; (2) may 

 have come from a single egg with a double yolk, or (3) from an 

 ovarial egg with two nuclei (cf. Franque, 1898, Stoeckel, 1899,. 

 H. Rabl, 1899, and von Schuhmacher u. Schwarz, 1900). It is 

 conceivable that from such an egg as this last, two blastodermic 

 vesicles and two chorions could develop within one zona pellucida, 



