48 On Double Monstrosity in Fishes. 



M. de Quatrefages, however, states that such was not the case in 

 two instances observed by him.] 



On the 19th February, nearly a month after the specimen 

 came into the possession of M. de Quatrefages, and about six 

 weeks after its exclusion from the egg, the two embryos were 

 close together and ready to unite on one side of the abdomen, 

 w^hilst on the other they were still separated by a considerable 

 space occupied by the vitellus. When exhibited to the Academy, 

 the vitellus was nearly gone, and the larger of the two was 

 nearly ready to feed. It had originally been placed to the right 

 of the vitellus, but had become superior, lying somewhat across 

 the smaller and more deformed individual, which it carried about 

 with it. 



The conclusion at which M, de Quatrefages arrives with re- 

 gard to this monster is, that it is formed by the coalescence of 

 two originally distinct embryos, and that the vitellus from which 

 it was developed was also double, the point of junction being in 

 his opinion indicated by the deep notch already mentioned at 

 the anterior part of the vitellus. This opinion is also shared by 

 M. Serres. 



M. Coste however maintains that there is only a single vitellus 

 and umbilical vesicle, since at whatever period these monsters 

 may be observed, the vesicle is always simple ; and this view is 

 further supported by the intimate connexion between the circu- 

 latory systems of the two embryos. He considers that as the 

 external lamina of the umbilical vesicle really forms a common 

 abdominal wall enclosing the vitellus, it is impossible to regard 

 the two embryos as distinct individuals developed at the poles of 

 a double vitellus and becoming coalescent at a late period of 

 their growth, as they are actually united from the first into a 

 single organism by this membrane, and their subsequent union 

 is only effected by the gradual contraction of this membrane as 

 the vitellus becomes absorbed. This conjugation is consequently 

 a primordial phainomenon, and of a much more intimate nature 

 than one of simple adherence, such as the case cited by M. Geof- 

 fi'oy Saiut-Ililaire, in which two chickens, hatched from two 

 separate yelks contained in the same egg, were found to adhere 

 to one another by the belly. 



M. LcrcbouUct holds the same opinion with I\I. Coste, regard- 

 ing the primitive siuiplicity of the vitellus, and his obsei'vations 

 (made upon the eggs of the Pike) appear to be tolerably con- 

 clusive upon this point, lie states that in the eggs of the Pike 

 the development of the embryo commences at the moment when 

 the blastoderm has almost completely enclosed the vitellus, by 

 the formation of a small triangular tubercle on the blasto- 

 dermal ridge [bourrelet blastodennique), and that from this 



