TRIPLE MONSTROSITY— RECORDS IN FISHES 35 



OTHER INSTANCES OF TRIPLICITY IN FISHES. 



1. Esox hums. An egg, observed four and a half days after fertilisation, showed two 

 embryos uniting posteriorly behind the pectoral region, and diverging widely in front. The 

 left component was normal, but the right one had two heads, the outer of which was somewhat 

 defective. In later development the latter showed only a single eye, viz. its outer one. The specimen 

 lived ten or eleven days and developed two hearts, one belonging to the left embryo and the other 

 to the (doubled) right component. The two hearts were independent as regards pulsation 

 (Lereboullet, US 1863 p. 217, PI. III. figs. 24-25). 



2. Sal mo far io. This is a blastoderm showing three embryonic rudiments. Rather more than 

 half of the yolk still remains uncovered. Two of the rudiments are extremely close to one another, 

 and, indeed, are in process of uniting posteriorly. The third is at an angular distance of approxi- 

 mately 60° to the left along the edge of the blastoderm. This is the youngest triple Salmonid of 

 which we have any record. Fuller development would no doubt have produced a form closely 

 resembling the preceding (Eauber, 202 6 p. 145, PI. VIII. fig. 15). Text-fig. 5 on p. 56 of this 

 work illustrates the specimen just described. 



3. Salmo salvdinus. Also a very early example. Only 18 to 20 mesoblastic somites are present, 

 and the yolk is not yet completely covered. The embryonic axis, which is single posteriorly, 

 bifurcates in front, and the right branch again divides (Klaussner, 123 p. 31, Taf. VI. fig 38). 



4. Salmo salvdinus. Similar to the preceding, but much older. The main bifurcation extends to 

 near the pectoral region. The second bifurcation affects the left component, and only reaches to the 

 optic lobe region (Klaussner, 123 p. 32, Taf. VI. fig. 40). 



5. Salmo fario. A specimen in which the absorption of the yolk is well advanced. It 

 belongs to the same type as the preceding ones. The main bifurcation reaches behind the 

 pectoral region, a pair of inner pectoral fins being present in its angle. The anterior bifurcation 

 affects the right component from the pectoral region forwards. Of the twin heads produced by this 

 latter bifurcation, the right one is reduced and on the way to becoming a parasite on its neighbour 

 (Klaussner, 123 p. 33, Taf. VII. figs. 42-43). 



0. Salmo fario. Also at a stage well advanced as regards absorption of the yolk. An 

 apparently simultaneous tripartite division takes place just behind the pectoral region. Inner 

 pectoral fins are present in both angles thus produced (Klaussner, 123 p. 34, Taf. VII. figs. 44-45). 



The preceding cases all belong to P>. 1 of the appended Classification Table (p. 38). 



7. Salmo fario. A moderately early stage. Two embryos are seen coming together at their 

 extreme posterior ends. The right one bifurcates just behind the region of the medulla. The 

 posterior ends of the two main components diverge in such a manner that one cannot consider them 

 as exhibiting axial union, though probably, had they survived, their tails would have remained united 

 by paraxial structures. Accordingly, the specimen belongs to B. 2 of the Classification Table on 

 p. 38. It is recorded by Klaussner (1:23 p. 39, PI. VI. fig. 39). 



8. Salmo fario. The specimen described in this paper. The axes of the two defective embryos 

 are united. These last, however, do not undergo true axial union with the principal embryo, being 

 united with the latter only by paraxial structures. The specimen is accordingly to be referred 

 to B. 2 of the table. 



9. Salmo fario. The verbal description of this specimen (Klaussner, 123 p. 34, PI. VIII. figs. 

 4G-47) bears out that there are three completely separated embryos, all being smaller than normal, 

 and one very markedly so. However, both of the illustrations seem to indicate union on the part of 

 the tails of two of the embryos. If the description is right, this specimen will belong to Division I. 

 of the Classification Table, and allowing for differences in the general conditions of development, will 

 be in line with those instances among mammals in which three separate embryos are found within 

 the same chorionic sac. On the other hand, if the figures are correct, the specimen will have an 

 important bearing on the question as to how far apart on the blastodermic margin embryonic 



