36 TRIPLE MONSTROSITY— DEVELOPMENT 



rudiments must appear in order that the growing embryonic axes may remain separate along their 

 whole length (see p. 7 and p. 30). 



10. Petromyzon plancri. The example of caudal triplicity recorded by Barfurth (S) in a larval 

 Petromyzon should be referred to here. The larva has three tails, each of which contains a spinal 

 cord, notochord, and caudal artery, as well as muscle plates and skin. Barfurth, however, considers 

 that probably two of the tails originated by a process of supra-regeneration after injury. If this be 

 so, the specimen is not properly referable to B. 1 of the table, which includes examples of true 

 posterior axial triplicity occurring in the mammals alone. 



11. Salmo fario. Klaussner describes a remarkable instance of anterior quadruplicity. The 

 posterior end is single. The main bifurcation extends to behind the pectoral region, and each 

 of the components is divided as far back as the optic lobe region. The four heads are all defective, 

 being unprovided with eyes (Klaussner, 133 p. 32, Taf. VI. fig. 41). 



12. Schmitt ( 316 p. 37) mentions the occurrence of several triple forms and of one quadruple 

 form in the set of 900 eggs (Salmo fario) referred to on p. 1 as having provided 68 double 

 monsters. Descriptive details are, however, awanting. 



MODE OF DEVELOPMENT. 



Regarding the mode of growth, and the causation of triple monsters, little can be added 

 to what was said under these headings in the part dealing with double monstrosity (pp. 6-8). 

 All the available evidence goes to show that double and triple forms arise in essentially the same 



manner, that is, by the appearance of different centres of embryo- 

 formation at a greater or less distance from one another on the 

 margin of the blastoderm. The example (Instance 1, above) 

 recorded and illustrated by Lereboullet, and the other early 

 examples noted by Ilauber (see Text-figure on this page) and 

 Klaussner leave no room for doubt on this matter. We are 

 speaking here, it will be understood, of anterior duplicity and of 

 union by the yolk-sac only (anadidymus and anakatadidymus), no 

 triple form showing mesodidymus or katadidymus having been 

 put on record. The heads invariably point in one direction, as 

 occurs also in the double monsters, for a precisely similar reason 

 (p. 7). 



f,g. . 5 .-Af.er Rauber ( m 6, Taf. vi it. U wU1 be remembered from the part dealing with double 



fig. 14). Egg of trout sixteen days after monstrosity (p. 7) that two embryonic rudiments which are to 



fertilisation, showing three embryonic nidi- .. > » ,, , , 



ments which are slightly unequal in size remain entirely separate from one another need to arise at a very 

 S^y^ ( ^ C L^SSS*SS2 considerable angular distance (approx. 165") from one another on 

 apart along the margin of the blastoderm, the edge of the blastoderm. If the description attached to 



The specimen is described under (2) (p. 35). t- . #» * i ■ 1 • f -l t -ii 



Instance 9 of triple monstrosity in fishes be literally followed, our 

 estimate of the distance in question will have to be reduced at least to 120°. On the other 

 hand, if the illustrations in connection with Instance 9 are correct, they supply evidence that 

 the previous estimate was not far from being true. 



Triple monster fish showing simultaneous tripartite division are possible forms. Instance 6 of 

 the preceding series may have been an example. The mode of formation could be explained by 

 assuming that the two outer centres of embryo formation appeared at an equal distance from the 

 middle one on the edge of the blastoderm. The chances are that this will happen very rarely indeed, 

 while, even when it does happen, there is always the likelihood that the egg as a whole, or one or other 

 of the components, will not survive long enough to give the monstrosity a chance of being observed. 



There do not appear to be any records, ancient or modern, indicating survival, or reputed 

 survival, of a triple monster fish beyond the embryonic period. Even as regards that period we still 

 know of examples only from Esox and the Salmonidae. 



