Harris Hawthorne Wilder 391 



has seen it in lizards and Panum describes separate embryonal areas upon 

 one yolk (hen's egg). See also Kaestner's figure of a double egg of 

 Pristiurus, 98. [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 hardly 

 probable, however, that duplicate twins would arise from an ovarial egg 

 with two nuclei, since in such a case the fertilization could be effected only 

 by means of two spermatozoa, thus introducing different paternal charac- 

 ters ; but if we reject all of Schultze's alternatives and substitute the pos- 

 sibility suggested above, that of the complete separation of the two l)las- 

 tomeres resulting from the first cleavage of a fertilized egg. the two 

 components would still remain within one zona pellucida and would 

 later become enclosed within a single chorion, which would develop a 

 single placenta to which each allantois would later become 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 embryonal areas. If far enough apart from 

 one another, each would develop its own amnion, but if near together a 

 common amnion would result, thus producing the condition given in 

 Case IV. This whole matter of the actual condition of the development 

 of two closely associated embr3'os is very obscure, as there are but scattered 

 and insufficient data bearing upon the case. It will receive a more 

 extended consideration later on, under the headings " Origin of com- 

 posite monsters " and " Other recent theories concerning the genesis of 

 composite monsters."] 



Case IV. — Similar to (III), but with both embryos enclosed in a 

 single amnion. This is a very rare case, explicable only by postulating a 

 single blastodermic vesicle upon which the two embryonal areas are nearly 

 or entirely in contact with one another, a case which has l)een described 

 by several authors as occurring in the hen's egg. In such a case there 

 would be an almost irresistible tendency towards the fusion of the two 

 embryos along the line of mutual contact, thus producing some form of 

 composite monster. (Schultze says: " Doppelmissbildungen,' ' but I use 

 the word double in a more restricted sense as explained below.) 



[As Case II is seen to be a variation of Case I with the two embryos 

 nearer together, so Case IV is seen to be a similar variation of Case III, 

 with a similar result, i. e., the more complete fusion of parts, although 

 here, owing to the direct connection of the two embryos the fusion is 

 liable to extend also to these and produce abnormal results. There are 



