LIMB F lELD OPERATIONS 287 



DISCUSSION : 



The time at which the antero-posterior axis of the limt field has been irreversibly 

 polarized has pushed hacl!: to the slit blastopore sta^e (Detwiler, 1955). The dorso-ventral 

 axis undergoes permanent polarization during stages #55 and #5^1. The relationship of the 

 ectoderm and the mesoderm in partial transplants suggests that the specific form of the 

 limb is probably contained within the mesodermal portion of the limb-field while the ecto- 

 dermal covering maintains a passive relation to the growth pattern of the underlying meso- 

 derm. The fusion of transplants, splitting and twinning of limbs, altering of axeSj and 

 the relation of the girdles all Indicate that the fundamental limb forming material is 

 primarily mesodermal. 



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