510 DEVELOPMENT OF PRIMITIVE BODY FORM 



the limb" (Swett, '23). As demonstrated by Harrison ('18) half discs (half 

 fields), left intact in the developing embryo or removed and transplanted to 

 other areas, develop into normal limbs. 



The above experiments of Harrison, together with those of Detwiler ('29, 

 '33) suggest that while the limb field is irreversibly determined at an early 

 stage to form limb tissue, the exact determination of the various parts within 

 the field is absent at the earlier phases of development. One kind of precise 

 determination is present, however, for the first digit-radial aspect (i.e., the 

 pre-axial aspect) of the limb appears to arise only from the anterior end of 

 the field, whether the field is allowed to develop intact or is split into two 

 parts. That is, if it is split into two portions, the anterior extremity of the 

 posterior portion, as well as the original anterior part of the limb field, de- 

 velops the pre-axial aspect of the limb. This antero-posterior polarization is 

 present from the first period of field determination. On the other hand, the 

 dorso-ventral polarity is not so determined; for if the transplanted limb disc 

 is rotated 180 degrees (i.e., if it is removed and reimplanted in its normal 

 place dorsal side down) it will develop a limb with the dorsal side up but 

 with the antero-posterior axis reversed (Harrison. "21). In these cases the 

 first digit-radial aspect will appear ventral in position. This result indicates 

 that the pre-axial aspect of the limb becomes oriented always toward the ventral 

 aspect of the limb. However, the experiments of Swett ('37, '39, '41 ) tend to 

 show that the reversal of the dorso-ventral axis occurs only when implanted 

 below the myotomes; for when the rotated limb field is implanted in the 

 somitic (myotomic) area, it will remain inverted. Factors other than those 

 resident within the limb field itself, probably factors in the flank area, appear 

 thus to induce the normal dorso-ventral axis when the limb disc is implanted 

 in its normal site. 



In the descriptions given above, the importance of the somatic layer of 

 mesoderm as the seat of the limb-forming factors is emphasized. It is obvious, 

 however, that the epidermal covering of the limbs derived from the epidermal 

 tubulation also is important in limb formation. For example, epidermal im- 

 portance is suggested by the experiments of Saunders ('49) on the developing 

 limb bud of the chick wherein it was found that the apical ridge of ectoderm, 

 located at the apex of the early limb bud, is essential for normal limb 

 development. 



Individual, or specific, organ-forming fields which appear in the gastrula 

 and early tubulated embryo thus are generalized areas determined to form 

 individual organs. As development proceeds, two main limitations are im- 

 posed upon the field: 



( 1 ) The cellular contribution of the field actually entering into the organ 

 becomes restricted; and 



(2) specific parts of the field become progressively determined to form 

 specific parts of the organ. 



