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MISCELLANEOUS. 



Investigations on Eggs with a Double Germ, and on the Origin of 

 Double Monsters in Birds. By M. C. Dareste. 



The coexistence of two embryos upon a single vitellus, indicated 

 by Wolf in the last century, has since been repeatedly noticed. The 

 author considers that the facts observed, although not numerous, 

 belong to two phenomena of very different nature, origin, and phy- 

 siological starting-points. 



Sometimes, during the first days of incubation, two distinct blasto- 

 derms, completely separated from each other, and each presenting 

 its transparent area, are observed. Subsequently these blastoderms 

 become united by the margins and form a single blastoderm, which, 

 however, is the result of the fusion of two primitively distinct blasto- 

 derms. Each transparent area may then give origin to an embryo, 

 and each embryo may envelope itself in its proper amnios. The two 

 embryos thus remain completely separated, being only mediately 

 united by the vitellus ; a second mediate union may also be effected, 

 subsequently to their formation, by the fusion of the vascular areas, 

 where these meet. 



In the second case there exists only a single blastoderm, and in 

 this a single transparent area, which is remarkable, however, for its 

 irregular form. The two embryos which are developed in this 

 single but irregular area give origin to a single vascular area (which, 

 however, is formed, at least partially, of the elements of two normal 

 vascular areas), and they become enveloped by a single amnios. 



The two embryos thus developed upon a common transparent 

 area remain in some cases completely isolated, except as regards 

 the indirect union effected by the vitellus. Then both of them may 

 be sometimes constructed normally ; sometimes one of them is imper- 

 fectly developed and forms an acephalous monster. In other cases 

 the two embryos unite directly and produce a double monster ; and 

 this union may be either early or late. 



The origin of these two modes of coexistence of two embryos upon 

 a single vitellus is very evident. In the first case the egg contains 

 two distinct cicatriculse before incubation ; in the second, only one. 

 The physiological consequences of these two arrangements are very 

 remarkable. It is no longer supposed that double monstrosity is 

 the result of the fusion of two embryos developed upon distinct 

 vitelli, and it is admitted that the coexistence of two embryos upon 

 a single vitellus is the starting-point of all cases of double monstro- 

 sity. The author goes still further, and maintains that, for the form- 

 ation of a double monster, the embryos must actually originate upon 

 a single transparent area, or, in other words, in a blastoderm pro- 

 ceeding from a single cicatricula. But it remains to be ascertained 

 why in some cases the two embryos are developed separately, whilst 

 in others they form a double monster. 



This question, moreover, is connected with another more general 

 one. Is this single cicatricula, which gives origin sometimes to two 

 distinct embryos and sometimes to two united ones, really simple 



