CHAPTER XI 



REGENERATION IN EGG AND EMBRYO 



NOT only do adult organisms have the power of regeneration, but 

 embryos and larval forms possess the same power, and even portions 

 of the segmenting, and also the unsegmented, egg may be able not 

 only to continue their development, but in many cases to produce 

 whole organisms. Haeckel observed in 1869-1870 that pieces of the 

 ciliated larvae of certain medusas, and even pieces of the segmented 

 egg, could produce whole organisms. The more recent experiments 

 of Pfliiger ('83) and of Roux ('83) on the frog's egg mark, however, 

 the beginning of a new epoch in embryological study. The expla- 

 nation of this is to be found, I think, not only in the introduction of 

 experimental methods, but also in the fact that Pfliiger and Roux 

 realized the important theoretical questions involved in their results. 



Pfliiger's experiments were made by changing the conditions 

 under which the egg develops in order to determine what factors con- 

 trol the development. Since these experiments were made with whole 

 eggs, the problems of regeneration were not directly involved in his 

 results, although his conclusions are of great importance in connec- 

 tion with questions concerning the regeneration of the egg. A part 

 of Roux's work dealt directly with the development of a new organ- 

 ism from a piece of the egg or of the embryo. Roux's principal dis- 

 covery l ('88) was that a half-embryo develops from either of the first 

 two blastomeres of the frog's egg, if the other blastomere has been 

 injured or destroyed, but that subsequently the missing half of the 

 embryo is " post-generated." Roux was led to this experiment by his 

 discovery that the plane of the first cleavage of the egg corresponds 

 very often to the median plane of the body of the embryo. 2 This 

 relation suggested that there might be some causal connection between 

 the two phenomena in the sense that the first cleavage plane divides 

 the material for the right side of the body from that of the left side. 

 In a descriptive sense this would be, of course, true if the two planes 

 do really correspond, and if there was no later shifting of material 



1 Roux's earlier experiments in 1885, in which the unsegmented or segmented egg was 

 stuck and a part of its contents removed, the remaining part making a whole embryo, will 

 be considered in another connection. 



2 This had been first discovered by Newport in 1851. 



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