No. 518] 



REGENERATION 



105 



But to return to the original question as to what should 

 guide us in deciding where, in regeneration, mere ab- 

 normalities or monstrosities end and where "palin- 

 genesis" and "atavisms" begin. Let us see how Davy- 

 doff proceeds with this question. He records among 

 others two cases where occasionally double structures re- 

 generate instead of the normally single structure, viz., 

 the doubling of the pericardial sac and of the notochord. 

 The meaning of these parallel facts is equally puzzling 

 to us and, broadly considered, the two facts are to all in- 

 tents and purposes of the same importance. But Davy- 

 doff rummages through the volumes in the library for 

 light upon the significance of these extraordinary in- 

 stances. There he discovers that the much-esteemed, 

 and certainly authoritative writer of the "Boitrago zur 

 einer Trophoecelthorie " in speaking of the "Herzblase" 

 of Balanocjlossus suggests in parentheses and under the 

 auspices of a question mark that the "Herzblase" may 

 have been primitively a double structure. This sugges- 

 tion is expressed just in two words— (urspriinglich 

 paarige?). Evidently DavydofT had not succeeded in 

 finding in the literature* another similar hypothesis that 

 the notochord, too, may have been primitively a double 

 structure. I judge so, because without offering any 

 further reasons, he dismisses the puzzle of a double 

 notochord by proclaiming it an abnormality, while honor- 

 ing the double pericardial sac with the distinction of 

 atavism. 



Thus does DavydofT solve the question, and whatever 

 merits or demerits his answer may have from the point 

 of view of common sense, one thing is certain: that there 

 is no direct, immediate way of deciding the question. 

 We must fall back upon a theory, a hypothesis, a sug- 



