ORIGINS OF EMBRYONIC PATTERNS 693 



In various experiments two individuals or organ systems developing 

 side by side with cellular continuity between them very generally show 

 opposed symmetry or asymmetry patterns. Even two hydranths of Cory- 

 morpha developing close together become "dorsiventral" or bilateral in 

 relation to each other, and the mirror-imaging of amphibian appendages 

 has long been known (pp. 390-95). Apparently there is in these and other 

 similar cases a relation between the two members tending to make the 

 pair a symmetrical whole. If the asymmetry of the amphibian appendage 

 results from a physiological differential, these cases of mirror-imaging re- 

 ceive a simple interpretation ; the high or the low side of the differential 

 is common to both, or the differential in one may induce a differential 

 symmetrical to it in the other. 



Both morphological and physiological evidences of asymmetry appear 

 in the chick blastoderm of the head-process stage. The anterior end of the 

 primitive streak bends slightly to the left (Fig. 167); and in chorio-allan- 

 toic grafts from this stage pieces from the left side show greater develop- 

 mental capacity than those from the right, suggesting a differential in 

 physiological condition from left to right (Rawles, 1936). In later stages 

 the embryo undergoes torsion progressively from the head region pos- 

 teriorly, so that it finally comes to lie on the left side. This suggests a 

 growth differential, higher on the left side during torsion, and dye reduc- 

 tion gives evidence of a physiological asymmetry at these stages (p. 160 

 and Fig. 55). Reversal of direction of this torsion (heterotaxia) occurs 

 occasionally under natural conditions. In a brief abstract Gray, Dodds, 

 and Worthing (1940, Anat. Rec, 78, Suppl., p. 77) report 6 per cent hetero- 

 taxia in some nine hundred embryos. They also find that various optically 

 active d- and /-substances have opposite effects on frequency of hetero- 

 taxia and suggest that the lateral reversal of differential growth is due to 

 interference with normal metabolism or to an asymmetrical utilization 

 of, or sensitivity to, the optically active substances. 



Of the four embryos developing from a single armadillo egg, one pair 

 is connected with the right, the other with the left, placental disk. In a 

 study of symmetry relations, as indicated by anomalies in the integumen- 

 tary bands of scutes, it was found that members of a pair are more nearly 

 identical and exhibit interindividual mirror-imaging more often than indi- 

 viduals of different pairs (Newman, 19156, 1916). Mirror-imaging in 

 individuals of opposite pairs is interpreted by Newman as indicating the 

 primary bilaterality of the original embryo; mirror-imaging in individuals 

 of a pair, as indicating the secondary bilaterality of the primary bud, 



