VARIOUS KINDS OF HUMAN TWINS 13 



at a later stage of which two chorions could fuse. Von Kolliker 

 considers it more probable, however, that in such a case the egg 

 would develop two embryonic areas upon a single blastodermic 

 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 aUantois 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. Von 

 Kolliker has observed such cases in hen's eggs (but without fusion 

 of the allantoids). M. Braun has seen it in lizards, and Panum 

 describes separate embryonal areas upon one yolk (hen's egg). 

 See also Koestner's figure of a double egg of Pristiurus, 1898. 

 (This case seems to put us on the right track regarding the origin 

 of duplicate twins, especially since it is stated that the twins are 

 always of the same sex, and although observations of later physical 

 identity are wanting, it seems safe to assume it. It would seem 

 highly improbable, however, that duplicate twins would arise 

 from an ovarian egg with two nuclei, since in such case the fertili- 

 zation could be effected only by means of two spermatozoa, thus 

 introducing two paternal characters; but if we reject all of 

 Schultze's alternatives and substitute the possibility suggested 

 above, that of the complete separation of the two blastomeres 

 resulting from the first cleavage of the fertilized egg, the two 

 components would still remain within one zona peUucida and 

 would later become inclosed within a single chorion, which would 

 develop a single placenta to which each allantois would become 

 later attached. Each blastomere would undoubtedly form at 

 first an independent blastodermic vesicle, but the close association 

 of the two would readily tend toward a fusion of the contact 

 surfaces, thus forming a single vesicle upon the surface of which 

 are two embryonic areas. If far enough apart from one another, 

 each would develop its own amnion, but if near together a com- 

 mon amnion would result, thus producing the condition given in 

 Case IV. The whole matter of the actual condition of the de- 

 velopment of the two associated embryos is very obscure, as 

 there are but scattered and insufiicient data bearing upon 

 the case. It will receive a more extended consideration later 

 on, under the heading "Origin of Composite Monsters" and 



