300 Duplicate Twins and Double Monsters 



at some little distance from one another. In one case investigated by 

 V. Kolliker, the two deciduse 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 villi of the two embryos had grown. In addition to this, one 

 of the placentae was of the type known as a placenta marginata, caused by a 

 fold of the decidua. [This case 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 of nothing else.] 



Case II. — Two separate blastodermic vesicles enclosed in a single 

 decidua. Placentas 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 apparently results from the same general 

 cause, i. e., two sejjarate eggs, which are, however, implanted nearer 

 together. This would 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 greater 

 approximation of the two eggs to one another.] 



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

 placenta, in the middle of which the two cords meet and upon which the 

 umbilical vessels closely anastomose. These are enclosed in a single 

 chorion and covered by a single decidua reflexa. This case is said by 

 Hyrtl to be more frequent than (I) and (II) but is not as frequent 

 according to Spath. The twins are always of the same sex. Schultze 

 says that the explanation of this singular condition is " zweifelhaft," and 

 gives the following possible explanations: (1) At first two chorions, as 

 in (II), the contact wall between which becomes absorbed later; (2) may 

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

 egg with two nuclei (cf. v. Franque, 98, Stoeckel, 99, H. Eabl, 99, and 

 V. Schuhmacher u. Schwarz, 00). It is conceivable that from such an 

 egg as this last two blastodermic vesicles and two chorions could develop 

 within one zona pellucida, at a later stage of which the two chorions 

 could fuse. V. Kolliker considers it more probable, however, that in 

 such a case the egg would develop two embryonic areas upon a single blas- 

 todermic vesicle and that a single chorion would then be the natural 

 result. Each embryonic area would develop its own amnion. In this 

 case the two allantoides would necessarily fuse, being included in a single 

 chorion, and there would come to be between the two embryos a single 

 (common) yolk-sac with two yolk-stalks. V. Kolliker has observed such 

 cases in hen's eggs (but without the fusion of the allantoides). M. Braun 



