LIMB BUD AN ILLUSTRATION OF FIELD CONCEPT OF DEVELOPMENT 509 



fuse together for a time, forming one continuous lateral body fold. On the 

 other hand, in the lungfishes (the Dipnoi) and in amphibia (the Anura and 

 Urodela), the appendage makes its first appearance, not as an elongated fold 

 of the lateral body wall, flattened dorso-ventrally, but as a rounded, knob-like 

 projection of the lateral body surface (fig. 227K-M). 



G. The Limb Bud as an Illustration of the Field Concept of Development 

 in Relation to the Gastrula and the Tubulated Embryo 



In Chapter 9 it was observed that the major presumptive organ-forming 

 areas are subdivided into many local, organ-forming areas at the end of gas- 

 trulation. In the neural and epidermal areas, this subdivision occurs during 

 gastrulation through influences associated with local inductive action. At the 

 end of the gastrular period, therefore, each local area within the major organ- 

 forming area possesses the tendency to give origin to a specific organ or a 

 part of an organ. The restricted, localized areas within each major organ- 

 forming area represent the individual, or specific, organ-forming fields. Dur- 

 ing tubulation, the major organ-forming areas with their individuated, 

 organ-forming fields are molded into tubes, and, thus, the individual fields 

 become arranged along each tube. Consequently, each tube possesses a series 

 of individual, organ-forming areas or fields, distributed antero-posteriorly 

 along the tube. 



As a result of the close association of cells and substances during gastrula- 

 tion and tubulation, many specific organ-forming fields are related to more 

 than one of the body tubes. Specific organ-forming fields, therefore, may have 

 intertubular relationships. For example, the lens field is located in the epi- 

 dermal tube, but, in many species, its origin as a lens field is dependent 

 upon influences emanating from the optic vesicle of the neural tube (see 

 Chap. 19). Another example of an association between the parts of two con- 

 tiguous tubes is the limb-bud field in the urodele, Amby stoma punctatum. As 

 the limb-bud field in this species illustrates various aspects and properties of 

 an organ-forming field, it will be described below in some detail* 



The presumptive anterior limb disc or limb field of Ambystoma is deter- 

 mined as a specific limb-forming area in the middle gastrular stage (Detwiler, 

 '29, '33). Later on in the embryo, it occupies a circular-shaped area within 

 trunk segments three to six. According to Harrison ('18) and Swett ('23), 

 its properties as a field, mainly are resident in the cells of the somatic layer 

 of the mesoderm in this area. If, for example, the somatic layer of mesoderm 

 in this area is transplanted to another area, a well-developed limb will result. 

 Also, the mesoderm of the dorsal half of the field forms a greater part of the 

 limb than the other parts, with the anterior half of the limb disc next in im- 

 portance. It appears, therefore, that the limb-forming potencies are greatest 

 in the dorso-anterior half of the limb field and become less postero-ventrally. 

 Moreover, not "all of the cells which are potentially limb forming go into 



