Limb and Girdle 



437 



ranged dorsoventrally. Mollier (1895) inter- 

 preted these layers as derivatives of the 

 myotomes, but Romer finds that they are 

 completely separated from the myotomes at 

 this stage and he is certain that their sub- 

 sequent history is independent of the 

 myotomes. 



In his stage II (Figs. 155 A-F) Romer 

 shows the rapid differentiation which has 

 occurred in the limb and girdle with a 

 slight increase (1 mm.) in length of the 



polarized at different times in development. 

 In Lacerta with the very rapid differentia- 

 tion one might expect that the axes are not 

 separable by time. 



LIMB AND GIRDLE DEVELOPMENT IN 

 BIRDS AND MAMMALS 



Normal development of the limbs and 

 girdles is well described in detail in birds 

 and mammals (Hamilton, '52; Patten, '51; 



Fig. 156. Maps showing the approximate areas for tissues of the future wing parts in stages 4 through 7. The 

 arrows indicate the direction of the future long axes of the wing parts designated. (From Saunders, '48.) 



embryo. There is a regional blocking out 

 of the heavy condensing mesenchyme. The 

 halves of the girdle are remote from each 

 other and no dermal elements are present. 



When the embryo has reached stage III 

 (Figs. 155 G-7), the girdle and the limb 

 parts are plainly in evidence. The rapidity 

 of limb growth and of its differentiation are 

 both much greater than one would find in a 

 similar stage in Amblystoma. 



Lacerta raises interesting problems; first, 

 because of the simultaneity of development 

 of the girdle and the limb; second, be- 

 cause of the three layered composition of 

 the mesenchyme which gives rise to these 

 structm-es; and third, because of the speed 

 with which differentiation in the whole 

 appendicular skeleton occurs. One would 

 expect results quite different from those 

 obtained in Amblystoma, particularly with 

 reference to the polarization of the limb 

 axes. In Amblystoma the three axes are 



Bardeen and Lewis, '01; Broman, '11). There 

 are, however, some experimental findings 

 which demand attention. Hamburger's ('38) 

 implants of the wing primordium into the 

 coelom (cf. Rudnick, '45) showed its capacity 

 for complete self-dififerentiation. By marking 

 with carbon particles Saunders ('48) has de- 

 termined the sequential order of the develop- 

 ment of the tissues of the wing and their 

 relation to its future parts. 



Almost all of the wing bud, at stage 4, 

 consists of the materials which will form 

 the proximal parts of the wing and its girdle. 

 On the basis of his experiments in which 

 the outgrowth has been carefully observed 

 and described, Saunders concludes that de- 

 velopment of the limb parts is in proximo- 

 distal order and that this is controlled by 

 the ectodermal cap of the wing primordium. 

 The results of the carbon marking experi- 

 ments are confirmed by the removal of the 

 ectodermal cap in successive developmental 



