Anatomy of Symmetrical Double Monstrosities in the Trout. 129 



cent, to 100 per cent., which can possibly be explained by some cause 

 other than the combination of acid with water. 



The contractions show that these points of discontinuity, though to 

 some degree real, yet to another degree are ideal in that there is within 

 the limits of 1 to 2 per cent, in the vicinity of such points a transition 

 stage. 



The values for fi are further expressed in terms both of Gladstone 

 and Dale's, and of Lorentz' formula, and it is shown that the values in 

 neither case are constant, but decrease with increase of concentration, 

 and also that Pulfrich's formula which expresses the relation between 

 the refractive index and the contraction in terms of a constant is 

 only approximately applicable for results differing by small per- 

 centage concentrations, but not so in the case of considerable 

 differences. 



The results are illustrated by a selection of curves, with especial 

 reference to the points of discontinuity. 



' The Anatomy of Symmetrical Double Monstrosities in the 

 Trout." By James F. Gemmill, M.A., M.D., Lecturer in 

 Embryology and University Assistant in Anatomy, University 

 of Glasgow. Communicated by Professor Cleland, F.K.S. 

 Eeceived February 6, — Eead March 7, 1901. 



(Abstract.) 



This paper contains the results of an investigation into the anatomy 

 of a series of trout embryos exhibiting different degrees of symmetrical 

 duplicity, and gives an account of the structural details which attend 

 the fusion, disappearance, or special adaptation of parts in the region of 

 transition from the double to the single condition. Some general 

 questions suggested by these results are also discussed. 



The monstrosities examined were four months old counting from 

 the time of fertilisation, and they form a fairly complete series ranging 

 from specimens in which the duplicity does not affect more than the 

 anterior part of the head to specimens in which there is union by the 

 posterior part of the body or by the yolk-sac only. The classification 

 adopted has special reference to the material at my disposal and is on 

 the same general lines as that given by Professor Windle in the ' Pro 

 ceedings of the Zoological Society,' 1895. 



The examination of the monstrosities was necessarily preceded by an 

 investigation into the anatomy of normal trout embryos at correspond 

 ing stages in development. The results of this investigation are 

 briefly given, special attention being paid to the cranial, visceral and 

 vertebral skeleton, which at this period is wholly cartilaginous. 



