438 



Special Vertebrate Organogenesis 



stages, which results in suppressing the 

 apical growth zone. The earlier the removal 

 of the ectodermal cap, the greater the de- 

 ficiency in wing parts. 



Saunders rightly interprets his findings 

 in stating that the apical ectoderm is essen- 

 tial. He draws, however, too great a con- 

 trast with the results obtained in Am- 

 blystoma. His statement that previous work- 

 ers have assumed a completely passive role 

 for the ectoderm is not justified, since in no 

 case was the role of the ectoderm important 

 to the problems under consideration. Harri- 

 son ('18) first showed that when the limb 

 mesoderm was removed and the ectoderm 

 which before formed the cover to the limb 

 mesoderm was used as a cover for the 

 wound, no limb development occurred, al- 

 though if the wotmd was left uncovered 

 limb development frequently took place. In 

 the light of this experiment in which meso- 

 derm which possessed the potentiality to 

 form limb was covered by ectoderm which 

 would have formed the covering of the 

 limb, it hardly seems possible that the same 

 conditions prevail in the essentiality of the 

 ectoderm. This is likewise shown in trans- 

 plantation of limb mesoderm to the flank 

 with a covering of flank ectoderm, for the 

 limb develops with its normal asymmetry 

 if the mesoderm is dorsodorsal in orienta- 

 tion and has reversed asymmetry if oriented 

 dorsoventrally. 



The flank ectoderm has dorsodorsal orien- 

 tation and, if directive, should give the re- 

 sult which Saunders obtained in transplant- 

 ing the wing primordium with inverted 

 orientation to the hindlimb region. The ini- 

 tial asymmetry was maintained. As Saunders 

 himself states, more evidence is needed on 

 this point, and he has outlined the experi- 

 mentation which would give the added infor- 

 mation. 



Just where the mammalian limb is going 

 to fit is problematic but there are already 

 some leads to future work. Rothfels, in a pre- 

 liminary unpublished study, has removed 

 the limb primordium of the rat embryo at 

 stage 29 and has grown the limb rudiments 

 in tissue cultiu-e. Her results in a small 

 group of experiments showed the develop- 

 ment of recognizable cartilages representa- 

 tive of complete limbs with deficiencies, in 

 the main, confined to the digits. The defi- 

 ciencies were about the same whether the 

 ectoderm was present or had been removed 

 from the primordium before culture. 



More recently, Moscona ('51) performed 

 dissociation experiments using trypsin to 



separate the cells of chick limb buds after 

 removal of the dorsal and ventral ectoderm. 

 The dissociated cells were then cultured. 

 Chondrogenic and myogenic cells tended to 

 aggregate, forming an inner nodule of car- 

 tilage around which the layer of myoblasts 

 was formed. This is, of course, a far different 

 experiment from that of Saunders, with the 

 chick in place upon its substrate of the 

 yolk mass, but the conditions of the ex- 

 periment seem to throw some light on the 

 capacity of the dissociated cells to attempt 

 to complete their potential destiny without 

 the ectoderm acting in an essential manner. 

 The problems of the limb, regarded as 

 solved by all too many, are still with us. 

 The present paradoxical results will be 

 reconciled by those who will complete the 

 thoughtful analysis of future experimen- 

 tation. 



REFERENCES 



Balfour, F. M. 1878 A Monograph on the Devel- 

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Balinsky, B. I. 1931 Zur Dynamik der Extremi- 

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Banchi, Arturo 1904 Sviluppo degli arti ad- 

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Bardeen, C. R., and Lewis, W. H. 1901 Develop- 

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Braus, H. 1903 Versuch einer experimentellen 

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1904a Die Entwickelimg der Form der 



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-1909 Gliedmassenpfropfung und Grundfra- 



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