TERATOLOGY. 



975 



aliquando duo separati integri embryones fue- 

 rint, qui clein contingentes et compress!, par- 

 tibus eorum nonnullis destructis, aliis coalitis 

 et conimixtis, concrescerent in unum novum 

 compositum corpus ; ea vero, quae vel defectu 

 partinin vel insolita structura monstra sunt, 

 non ita fieri ut prius integri et naturales em- 

 bryones fuerint, qui deinde per causas acci- 

 dentales ad generationem non pertinentes 

 mutilati vel transmutati fuerint; sed necesse 

 esse, ut utraque monstroruw genera a primis 

 suis initiis jam ejusmodi monstra fuerint." 



If we are right in not admitting the exist- 

 ence and the fusion of two distinct germs, 

 we necessarily adopt the opinion that only 

 one germ has been formed, and that in this 

 excess of formative power lies the cause and 

 origin of every monstrous duplicity. Mr. 

 Allan Thompson demonstrated, in coincidence 

 with Wolff, Von Baer, and Reichert, as may 

 be seen in the wood-cut (./%. 629.), that 



Fig. 629. 



From a fowl's egg after sixteen or eighteen hours' in- 

 cubation. Magnified four times, 

 a, the germinal area of the cicatricula ; b, the 

 transparent area, containing two primitive traces 

 of embryos ; c c, primitive grooves of the double 

 embryonic trace, on each side of which are seen 

 the laminae dorsales. 



(After A. Thompson.) 



upon one yolk, and in one germinal mem- 

 brane or blastodermatic vesicle, there may be 

 formed, in birds, two primitive grooves, which, 

 in their ulterior increment, shall probably form 

 a double monster, as may be seen in a goose's 

 egg, after five days' incubation, represented 

 after Allan Thompson, inj%.630. By the for- 

 mation of such a double primitive groove in a 

 single ovum, we may explain the origin of the 

 principal types of double monsters; and on 

 this point a recent observation of Valentin 

 seems particularly worthy of notice, viz. that 

 in which an injury, inflicted on the caudal ex- 

 tremity of an embryo on the second day, was 

 found, on the fifth, to have produced the 

 rudiments of a double pelvis and four in- 

 ferior extremities. But if we admit this cause 

 for those large and principal types, we must 

 acknowledge that it is insufficient for the 

 heteradelphs, and for all those cases in which, 

 the body remaining single, some parts are 



Double embryo removed from a goose's egg after five 



days' incubation. Magnified four times. 

 <7, the common heart ; h, rudiments of the superior, 

 t, of the inferior extremities ; k, the common ce- 

 phalic fold of the amnios ; I, the caudal folds. 

 (After A. Thompson.) 



double. For these the excess of formative 

 power is the only explanation we can give. 

 We understand, under this name, not the 

 nisus formativus of the ancient physiologists, 

 working as a Deus ex macfiina, but the physical 

 and vital metamorphoses materice, to which the 

 formation of a new being ought to be attri- 

 buted. Those who are fond of the modern 

 nomenclature may name it, if it pleases them, 

 typical or organic power. But enough, we admit 

 such a power, of whatever name it may be, 

 and contend further, that different degrees or 

 quantities of excess lie at the origin of all the 

 cases of double monsters, the degree of excess 

 determining in each the degree of duplicity. 

 If a certain excess of power be admitted 

 capable of producing any one case of duplicity, 

 other amounts of excess may be believed 

 capable of producing all the other cases which 

 differ from it only in degree ; and I meant to 

 have proved, in my monograph, and in the 

 succinct survey I have given of double mon- 

 sters in this Article, that all double monsters 

 may be referred to differences in degree of 

 deviation from the normal singleness. 



But in many double monsters we see excess 

 in one part and defect in another, so that we 

 must suppose, in our hypothesis, that in these 

 cases the power, more or less excessive in 

 quantity, is also wrongly distributed. Nor 

 is this inconceivable ; for since, in the normal 

 developemental power, we must imagine at 

 least two elements, quantity and distribution, 

 and must acknowledge that, for the attain- 

 ment of a perfect result, the quantity must be 

 distributed in definite proportions to each 

 part, so it is not improbable that, in certain 

 circumstances of fault in the ovum, a normal 

 or an excessive power may be distributed dis- 

 proportionately in the several parts. 



To sum up, therefore, our reasons for re- 

 jecting the hypothesis of fusion of ova in 



