4 DOUBLE MONSTROSITY— CLASSIFICATION, CAUSATION 



Class III. Union in pectoral region, adjacent pectoral fins not being present. 



Class IV. Union in pectoral region, adjacent pectoral fins being present, but united and 



reduced in size. 

 Class V. Union by the body or tail, the united portion ending as normally in a single 



symmetrical tail. 

 Class VI. Union by the body or tail, the united portion ending in a composite tail with 



various structures still doubled. 

 Class VII. Union by the yolk-sac only, or by the yolk-sac and the ventral edge membrane 



close behind it. 

 Class VIII. The condition of imperfect doubling exhibited by (a) the (hemi)mesodidymi and 



(b) (hemi)katadidymi (p. 25). 

 Class IX. Longitudinal or parallel union. 



Comparing this scheme with the one given by Windle it will be seen that 



Class I. corresponds (approximately) with 1-2 of Windle. 



„ HI- 



IV 



v 



VI 

 „ VII. 

 „ VIII.l 



„ ix. J 



With regard to parasitic forms, though it may be convenient for descriptive purposes to group 

 them together, as in Class 11 of Windle's scheme, they should more properly be distributed among 

 the other classes according to the attachment of the parasite. 



Schmitt's scheme and mine do not so readily fit into one another. From the practical point 

 of view the former is chiefly useful in the analysis of the forms which are grouped under Classes 

 V and VI. of the latter (pp. 20-23). 



It must be kept in mind that any scheme for the classification of anadidynii can only 

 aim at dividing up into convenient groups a series of forms which is essentially continuous and 

 unbroken, the members differing from one another by characters which depend mainly on the 

 distance which separated the component twins at the time of their first appearance on the margin 

 of the blastoderm. As will be explained under E (p. 6), these components are brought together 

 during the natural elongation of their embryonic axes, and unite in what is designated primary 

 fusion. On the other hand, it is probably as a result of the working of secondary fusion that 

 such discontinuities as can be recognised in the series take their origin — for example, that twins 

 united in the head region can always (so far as my experience goes) be grouped under Class I. 

 or Class II., the final union of their brain cavities being fixed either in the region of the optic 

 lobes or in that of the medulla. In the same way, though in a less striking degree, secondary 

 fusion has helped to mould the structural details in the succeeding groups as well as to accentuate 

 the external characters which have been employed above in their classification. Other references 

 are made to secondary fusion on pp. 11, 15, 30, 43. 



D. CAUSATION. 



There is strong reason for believing that the occurrence of double monstrosity is due in the 

 main not to environmental factors, but to conditions which are inherent hi the fertilised germ cell. 

 Lereboullet's {US) observations on the pike went to show that in spite of sameness of environment, 

 different broods of eggs give rise to different prevailing types, as well as to different propor- 



