340 



LAWS OF VARIATION. Chap. XXVJ. 



faces. Whenever two bodies or two heads are united, each 

 bone, muscle, vessel, and nerve on the line of junction seems 

 to seek out its fellow, and becomes completely fused with it. 

 Lereboullet, 2 who carefully studied the development of double 

 monsters in fishes, observed in fifteen instances the steps by f 



which two heads gradually became fused into one. In this and 

 other such cases, no one, I presume, supposes that the two 

 already formed heads actually blend together, but that the 

 corresponding parts of each head grow into one during the 

 further progress of development, accompanied as it always is 

 with incessant absorption and renovation. Double monsters 

 were formerly thought to be formed by the union of two 

 originally distinct embryos developed upon distinct vitelli ; but 

 now it is admitted that " their production is due to the sponta- 

 neous divarication of the embryonic mass into two halves;" 3 

 this, however, is effected by different methods. But the belief 

 that double monsters originate from the division of one o; 



does not necessarily affect the question of subsequent fusion, or 

 render less true the law of the affinity of homologous parts. 



The cautious and sagacious J. Miiller, 4 when speaking of 

 Janus-like monsters, says that " without the supposition that some 

 et kind of affinity or attraction is exerted between corresponding 

 "parts, unions of this kind are inexplicable." On the other 

 hand, Vrolik, and he is followed by others, disputes this con- 

 clusion, and argues from the existence of a whole series of mon- 

 strosities, graduating from a perfectly double monster to a mere 

 rudiment of an additional digit, that " an excess of formative 

 " power " is the cause and origin of every monstrous duplicity. 

 That there are two distinct classes of cases, and that parts may be 

 doubled independently of the existence of two embryos, is certain • 

 for a single embryo, or even a single adult animal, may produce 

 doubled organs. Thus Valentin, as quoted by Vrolik, injured 

 the caudal extremity of an embryo, and three clays afterwards it 

 produced rudiments of a double pelvis and of double hind limbs. 





2 ' Comptes Rendus/ 1855, pp. 855, * < Elements of Physiology,' En 

 1029 - translat., vol. i., 1838, p. 412. With 



3 Carpenter's 'Comp. Phys./ 1854, respect to Vrolik, see Todd's ' Cyclop, of 

 p. 480; see also Camille Dareste, Anat. and Phys./ vol. iv., 1819-52, 

 ' Comptes Rendus/ March 20th, 1865, p 973 



p. 562. 



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