394 PATTERNS AND PROBLEMS OF DEVELOPMENT 



Harrison regards the asymmetry of the Hmb as an expression of an 

 "intimate structure" and suggests an analogy to compounds composed of 

 asymmetric molecules. Przibram (192 1) postulates a space lattice which 

 is assumed to reverse its asymmetry as required by experimental data. 

 At present, however, there is no evidence of the existence of such an 

 oriented intimate structure or space lattice as the basis of axiate pattern; 

 and the mechanism of reversal of such a structure in a limb-bud trans- 

 plant by some effect of the host pattern, or in relation to another bud, 

 requires further hypothesis. Interpretation in dynamic terms seems to 

 offer less difficulty. There is an anteroposterior and a dorsiventral differ- 

 ential in susceptibility and in rate of dye reduction in the amphibian em- 

 bryo at stages corresponding to those used in limb transplantation. Pig- 

 mentation has made it impossible with the material at hand to demon- 

 strate a gradient in rate of dye reduction in the limb bud, but in the chick 

 limb bud such a gradient is clearly evident (Rulon, 1935). The limb pri- 

 mordium must share in the gradient pattern of the body, but the dorsi- 

 ventral differential in it, though apparently present in gastrula stages 

 (Swett, 1938c, 1939), can be altered by change in orientation with respect 

 to the host pattern up to much later stages. The dorsiventrality of a small 

 transplant is alterable up to a later stage than that of a large transplant 

 (Lovell, 1937). Larger limb-disk transplants of Triturus are more likely 

 to develop than smaller. With transplantation to the mid-ventral line 

 duplications are usually ulnar and in a plane transverse to, but on the 

 flank are usually radial and in the plane of, the longitudinal body axis. 

 Inclusion in the transplant of parts about the limb primordium, particu- 

 larly the region dorsal to it, results in better limb development (Takaya, 

 1938). Here, as in Hydra and other invertebrates, size of transplant ap- 

 pears to be a factor in its persistence, and the body pattern is apparently 

 a factor in determining not only plane of duplication but direction of 

 radio-ulnar asymmetry. 



The frequent rotations of limb-bud transplants in other than normal 

 orientation noted by various authors (e.g., Nicholas, 1924) suggest pres- 

 ence of effective dynamic factors. If such factors can determine rotation 

 in many cases, it is not difficult to conceive that they may determine a 

 physiological axiate pattern in a transplant that does not rotate. More- 

 over, since a dynamic factor of some sort is apparently required to bring 

 about reversal of an intimate structure or space lattice, it is at least a 

 pertinent question whether it is necessary to postulate the structure. As 

 regards duplication and mirror-imaging, the effects of two dynamic fields 



