REVIEWS — THE ORGANIC CYCLE IN ORGANIC NATURE. 519 



consequence. They both, however, have this in common, that the great end 

 of their existence is the multiplication of the race — an end to which the 

 nutritive and animal functions are always subordinated. 



We now pass to the fourth chapter, whence we must quote a 

 passage explanatory of the most recent speculation on the origin of 

 "double monsters." 



A single ovum has been observed to originate two distinct axes of embryonic 

 growth. Cases of a double primitive trace of organization have been met with 

 in the bird's egg, by Dr. Allen Thomson and others, and it is probably in some 

 such way that we may most feasibly account for the origin of what are termed 

 " double monsters."* At all events we have in these, as much as in the best 

 marked cases of alternation of generations, a production of two more or less 

 typical organisms from a single original germ ; for it is now generally agreed 

 that such monstrosities cannot be well explained on any supposition of the 

 fusion of two independent embryos. 



This conclusion rests principally on the following considerations : — 



1. In all such monsters the duplicated parts are connected together, and 

 derive their vessels from a common trunk ; we never find a face springing out 

 of the chest, legs implanted on the head, or any such mal-position of parts. 



2. Double monsters form a continuous series, in which the degrees and modes 

 of deviation from singleness gradually increase, and pass, without any abrupt 

 steps, from the addition of a single ill-developed limb to the nearly complete 

 formation cf two perfect beings ; so that no theory can be tenable that will 

 not account for the simpler as well as the more complete instances of duplicity 

 — that cannot explam, 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 all, 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, originally single, is farther 

 supported by an observation of Valentin'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. LerebouUet, according to whom, in particular species — aa 

 the Pike— the formation 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, 



• Edinburgh Monthly Joutn. Med. Science (1844), IV., pp. 479, 568, 639. See also Vrolik'S 

 article on Teratology, in Cyc. Anat. and Phys. 

 t Vrolik, Op. Oit. 



