CHAPTER II. 



TRIPLE MONSTROSITY. 



Description of a specimen, - ... - p. 33 



Other instances in fishes, - - - - p. 35 



Mode of development in fishes, - - ■ p. 36 



Appendix on Triplicity in other vertebrates, and on Classification in Germinal 



Triplicity, - - - - - • - p. 37 



DESCRIPTION OF A SPECIMEN. 



TmrLE monster fishes are extremely rare. In the trout, I have only come across one example, 

 as contrasted with over seventy double forms. In birds, on the other hand, Dareste's figures show 

 one example of triplicity to nineteen of duplicity, the latter condition in turn occurring on an 

 average in one out of every two hundred and fifty eggs incubated. 1 



My specimen came from a brood of young trout which had been hatched for about five 

 weeks, and were still subsisting on the egg-yolk. The general appearance is shown in PL XXII. 

 fig. 94. Figs. 95-99 in Pis. XXII.-XXIII. and Figs. 32-34 in PL VII. also have reference to this 

 specimen, and illustrate the alimentary canals, the Wolffian ducts, and various transverse sections. 

 It will be seen that only one of the component embryos is properly developed. In what follows, 

 this component will be designated the principal embryo. The other two, which may be called " A " 

 and " B," are much smaller, and are defective in various important respects. Posteriorly they 

 become united first with one another and then with the principal embryo in the manner to be 

 afterwards described. 



Principal Embryo. This embryo is slightly smaller than ordinary examples of the same age, 

 but is otherwise normal except in the posterior fourth of its length, where it becomes distorted 

 through the addition of tail elements belonging to the defective embryos. It retains along its whole 

 length its own spinal cord, notochord, neural and haemal arch cartilages, muscle masses, aorta, 

 caudal vein, and alimentary canal. 



1 It will be seen that in birds the proportion of triple to double monsters is thirteen times greater than the proportion of 

 double monsters to normal forms. This fact seems to indicate that blastoderms with two centres of embiyo formation show a 

 greater tendency to exhibit a third centre than can be accounted for by the theory of probability applied to the results of 

 simple random distribution. We may suppose that in ova generally there is a certain liability to the incidence of duplicity 

 or multiplicity. The result is to produce a certain proportion of cases in which there are at least two centres of embryo 

 formation. Within this latter group of cases it may naturally be expected that the incidence will show intensification if the 

 liability be fundamentally duo to a predisposition depending on inherent germinal factors. Daresto's figures indicate that 

 such an intensification is actually present, the final result being that the ratio between double and triple monstrosities is so 

 far lessened as to be approximately equal to the square root of the ratio between normal and double forms. 



In the trout we do not possess so complete a series of observations dealing with early stages, as is that of Dareste for 

 birds. However, Rauber (202 6 139-184), examining a large number of ova ten to sixteen days after fertilisation, obtained 

 one triple rudiment as compared with eighteen which showed duplicity. If we take I in 300 (see p. 1) as something like 

 the average ratio between double and normal forms, it will be seen again that this ratio is approximately equal to the square 

 of that indicated by Rauber's observations as existing between double and triple monstrosity. All this is in consonanco with 

 the view that the formation of double or multiple embryos has behind it something of the nature of germinal predisposition. 

 E 



