DOUBLE MONSTROSITY— HEMIDIDYMUS 27 



stimulation immediately behind the embryonic rudiment. In one example, the body-halves 

 commenced to separate in front of the first muscle plates, while in the other the separation began 

 at the anterior border of the auditory vesicles. An outer series of sixteen to eighteen muscle plates 

 was present in each component in both cases. At certain places small inner muscle plates were 

 found, while various portions of the notochords were absent or defective. The presence of inner 

 muscle plates in the one component corresponded, on the whole, with their absence in the same 

 region of the other component. There was similar mutual correspondence between normal and 

 defective formation of the notochord in the two components. 



Moser (165), who lays stress on these alternations, would seek to throw light on the structure 

 of hemididyini in general by supposing that three kinds of fission on the part of the growing 

 embryonic axis may take place, viz. (1) ordinary equal, producing similar components; (2) ordinary 

 unequal, producing components one of which is larger than the other; and (S) zig-zag, producing 

 components with certain alternating structures. 



References to minor abnormalities observed by the author, and traceable to mesodidymus 

 or katadidymus, will be found on pp. 52, 54 of the present work. 



Causation. Mesodidymus and katadidymus in fishes may be produced experimentally through 

 the same kinds of agencies as cause defective conditions in general. Thus Knoch (127) found 

 duplicity of the posterior kind to occur much more frequently in eggs hatched out in water which 

 was stirred up by an active current than in still water. Jablonowski (106) was able to produce 

 splitting of the embryonic rudiment by the action of common salt solutions. A similar result 

 occasionally appeared in Stockard's (236 p. 105) experiments with lithium chloride, and Kopsch 

 (132) induced the formation of two hemkhdymi by electrical stimulation. 



A satisfactory account of the mode of origin of hemididyrnus can, however, hardly be given at 

 present. Probably the condition may result from different factors. Before discussing these, it will 

 be necessary to refer again briefly to what is known regarding the earliest stages in the formation of 

 a normal embryo. 



Kopsch's experiments seemed to show that the embryonic blastema for the head and body 

 is spread out, to begin with, over a small segment of the blastodermic margin, the head blastema 

 occupying a middle position and being flanked on either side by the body blastema, which is 

 thus divided into two separate masses. Next, the two masses of body blastema come together 

 behind that of the head, and, having united, grow backwards. The region formed by their actual 

 union is called by Kopsch the primarily formed portion of the body, and we see that it has resulted 

 from a very limited sort of concrescence. It produces the rest of the body by backward growth, 

 and, as was stated on p. 6, during this growth material is drawn from the adjacent thickened 

 margin of the blastoderm for the formation of the lateral and ventral body-walls, and in particidar 

 for the muscle plates. 



On the other hand, Morgan (161) and Sumner (24.2) hold that the blastema for the whole 

 embryo is disposed from the first in a spatially single centre, and that this centre produces the 

 definite body and tail by backward growth, accompanied by utilisation of material from the 

 thickened adjacent margin of the blastoderm in the manner already noted. These authors, 

 accordingly, deny even the limited degree of concrescence which Kopsch would admit. 



If we adopt the view of Morgan and Sumner, we may look on hemididymus as capable of being 

 produced either (A) by factors which tend to draw apart into two diverging horns the backwardly 

 growing mass of blastema which should give rise to a normal body or tail ; or (B) by factors which 

 hinder the direct backward growth of the blastema in question, causing it to split, so to speak, into 

 two streams which flow round the obstacle ; or (C), by any of the causes, innate or external, which 

 tend towards excessive growth and fission on the part of a growing system. 



If we adopt Kopsch's view as to the mode of formation of the embryo we shall have to say in 

 addition that the causes detailed under (A) and (B) may operate at the very start by preventing 

 concrescence on the part of the masses of blastema from which the p) % imarily formed portion of the 

 body arises. 



