MONSTROSITIES. , 275 



were produced by fusion of two embryos, and the tlieory tliat attributed a separate 

 vitellus to each embryo was incorrect. He believed there was only a single germ, 

 but that this, by becoming developed in two directions, instead of one (as normally), 

 gave rise to two more or less distinct embryos. That the blastodermal ridge plays 

 a most important part in the formation of these embryos, and in fact constitutes 

 the " true embryonic germ, which is always simple and single, like the vitellus 

 which is covered by the blastoderm, but when its development is deranged from, 

 its regular course, is capable of vegetating like the substance of which the bodies 

 of polypes are composed, so as to produce various forms, which however, in their 

 subsequent development, always show a tendency to return to the original type 

 of the species." 



Vrolik likewise remarked, as an objection against the hypothesis of fusion of 

 two originally perfect and separate embryos, that double monsters " form one 

 series, among whose several members the degrees and modes of deviation from 

 singleness gradually increase, and pass without one abrupt stop from the addition 

 of a single ill-developed limb, to the nearly complete formation of two perfect 

 beings." He considered one germ being provided with an excess of formative 

 power becomes the cause and origin of every double monster. In fact, we do not 

 see fusion, but an excess or irregular distribution of developmental power, and 

 instances of singleness tending towards reduplication and not of reduplication to 

 singleness. 



Valentin concluded that an injury iuflictod on the caudal extremity of an 

 embryo on the second day, had been found on the fifth to have produced the rudi- 

 ments of a double pelvis and four inferior extremities. But Vrolik said, if we admit 

 this cause for those large and principal types, we must acknowledge that such is 

 insufficient to account for those cases in which, the body remaining single, some 

 parts are double, and here excess of formative power is the sole explanation we 

 can offer. In many double monsters there may be excess in one part and defect 

 in another, the power being more or less excessive in quantity and being also 

 wrongly distributed. " It is not impossible," observed Vrolik, " that excess of 

 power in the ovum, which all admit can alone explain the lower degi'ees of 

 duplicity, may, in proportionally higher degrees, perhaps by the formation of two 

 primitive grooves, produce the most complete double monster, or even two such 

 separate individuals as are sometimes found within a single amnion." 



The eyes may be modified iu various ways, for some fish are born blind owing 

 to entire absence of the eyeball, or it may bo present on one side of the head but 

 not on the other. In many cases the eyeball may be present, but in a more or less 

 rudimentary or abnormal condition. Or there may be a single eye situated on or 

 near the top of the head. Two eyes may be merged into one, or we may have 

 three eyes, or even four eyes on one head (plate xii, fig. 1 and la), while in such 

 as have additional heads the eyes may be normal in each, or, as in the example 

 figured (plate xii, fig. 11), one eye may be absent from one of the heads; while a 

 double optic nerve has been observed with a single eyeball, or even a single optic 



(6.) Sometimes three heads were present, one of these was thus described by M. Lereboullet. It 

 was a double embryo, composed of two bodies united behind, but quite free in front. One of these 

 bodies was of the normal form ; the other bore two heads, of which that on the left was of the 

 normal form and furnished with two eyes, while that on the right ouly bore the right eye, tlie 

 union of the two heads beiug effected at the point where the left eye ought to have been. This 

 embryo was still within the egg when described ; it had two hearts, one common to the two 

 principal bodies, situated at their bifurcation, tlie other placed iu the angle of union of the two 

 heads. He considered that two fillets had been formed, one of which had been terminated by 

 two cephalic lobes and acquired two furrows (as in no. 2), while the other contiued simple. 

 These two embryos thus united posteriorly (as in no. 1) producing an embryo with one tail, two 

 bodies, and three heads. 



(7.) When the development of the egg was retarded by means of a low temperature, the ridge 

 of the blastoderm produced no embryo, but contracted gradually like the opening of a bag, its 

 substance became condensed and formed a mammillated tubercle projecting from the surface of 

 the vitellus. This tubercle continued living, rose more and more from the surface, acquired a 

 linguate form, and at last constituted an elongated body, narrowed in front, divided transversely 

 into vertebral lamella;, witltout dorsal chord or sensitive organs, but furnished with a heart of which 

 the contractions were sometimes very lively. 



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