SYMMETRY IN TRANSPLANTED LIMBS 101 
The evidence which Morrill ('19) has collected from the study 
of double monsters in fish embryos shows that situs inversus does 
occur, but that it is the exception, not the rule, and that there is 
no ''ver}^ precise relation betw^een the amount of separation of 
the two components and the occurrence of mirror imaging. ""^ 
This would seem to oppose the view expressed above, that it is 
the proximity of two grow^th centers that causes the reversal of 
one. Still, in the absence of statistical data regarding the corre- 
lation of the two events, it is unsafe to draw a definite conclusion. 
In this connection the recent work of Spemann and Falkenburg 
('19) is of the greatest importance. By extension and modifica- 
tion of the earlier methods of the former, these investigators ob- 
tained a large number of twins in Triton by constricting the eggs 
in segmentation stages or in the early blastula. They found that 
in a large number of the cases one individual of a pair (the right- 
hand member in all cases but one) has complete situs inversus 
viscerum. Spemann, after an admirable critical analysis of the 
question, reaches the conclusion, that while some asymmetric 
intimate structure must be postulated to account for the normal 
asymmetry of the vertebrate body, there is no proof from these 
obliterated by the tertiary mirror-imaging between antimeric halves of the same 
individual, which latter is the prevailing symmetry system In gen- 
eral, mirror-imaging between individuals of opposite pairs is interpreted as an 
evidence of the early system of symmetry present in the embryonic vesicle 
before polyembryonic budding began. When the primary buds are formed they 
are the product of the antimeric halves of the undivided embryo and therefore 
should have mirror-image relations, but a partial physiological isolation of the twa 
buds permits a certain degree of reorganization or regulation in the symmetry 
relations, that tends partially to obliterate the original symmetry relations of 
the undivided embryo. Similarly, when each primary bud subdivides to form 
the secondary buds that are the primordia of the definitive individuals, a certain 
residuum of the primary bud symmetry system is carried over, manifesting itself 
in mirror-imaging between the twins derived from the same primary bud. But 
here again a certain amount of regulation occurs so that a third system of sym- 
metry, the bilateral symmetry of each individual, tends to obliterate former 
systems of symmetry." Newmann: op. cit., pp. 200-201. i 
"2 Tannreuther ('19, p. 359) has recently figured a double chick embryo in 
which the two individuals are united only by the posterior tip of the primitive 
streak. Although trunk and head are entirely separate, the heart of the embryo 
on the left shows situs inversus. No mention is made of this fact in the text. 
