IN THE EMBRYOGENY OF THE HIGHER ANIMALS. 159 



as the more complete instances of duplicity that cannot 

 explain, for example, the existence of superfluous limbs. 

 As M. Vrolik remarks, " the limbs are mere off-shoots, and 

 are produced at so late a period, that if we could imagine 

 two embryos to come in contact by their shoulders or pelvis, 

 and a fusion of those parts to take place, we should still 

 have to explain how one of them, leaving only an arm or a 

 leg behind him, could for the rest of his substance, head, 

 trunk, and ah 1 , wholly disappear." 



3. The two monsters are always of the same sex, which 

 we know, from the case of twins, is very far from being a 

 constant rule with associated embryos. 



The theory of the furcation of a germ or embryo, origi- 

 nally single, is farther supported by an observation of Va- 

 lentin's, that an injury inflicted on the caudal extremity of 

 an embryo on the second day was found on the fifth to 

 have produced the rudiments of a double pelvis and four 

 inferior extremities.* 



Reference may be made also to the observations on the 

 development of the ova of fishes by M. Lereboullet, accord- 

 ing to whom, in particular species as the Pike the for- 

 mation of such monstrosities may be determined at pleasure, 

 by placing the eggs in certain conditions unfavourable to 

 development. In this case the blastodermic ridge forms 

 on its surface two tubercles instead of one, and from each 

 of these an embryonic fillet is produced, the farther deve- 

 lopment of which gives rise to double embryos of various 

 kinds.*)- 



The detachment of a portion from the body of the ger- 

 minal mass has not unfrequently been observed in the em- 

 bryogeny of the Molluscan Gasteropoda. In this case the 

 isolated segments may become clothed with cilia, and re- 



* Vrolik, Op. Cit. 



f Annals of Nat. History, 2d Ser., XVI., 49. 



