DUPLICITY. XV 



Amongst the Vertebrata dichotomy of the axis is by no 

 means unknown ; it may occur at either extremity, but the 

 posterior extremity is most frequently affected. This con- 

 dition originates in dichotomy of the primitive groove and 

 notochord. Many discussions have been held -and much 

 difference of opinion exists as to its origin. That there is 

 but a single germinal area is undoubted* ; but the question 

 at issue is whether there were originally two grooves and two 

 notochords arising from independent sources (i. e. possibly 

 from two vesicles of Purkinje being present in a single ovumf 

 or from the original fusion of two yelks) or from an originally 

 simple ovum. The former hypothesis is unsubstantiated, 

 since the occasional presence of two vesicles of Purkinje, 

 or the fusion of two yelks in an ovum, which we do not 

 know will produce a double embryo, cannot be regarded in 

 the light of evidence ; and the hypothesis entirely fails to 

 account for the presence of a single blastoderm, a single 

 germinal area, or a dichotomous embryo : on the other hand, 

 the view that excess of formative power or nutriment pro- 

 duces a tendency to the complete or partial doubling of the 

 embryo upon a single blastoderm, by the formation of two 

 points of growth in the place of one, is borne out by 

 numerous facts. 



The doubling of a single limb, although excessively rare, 

 occurs at times (see No. 307), bearing out the view that a 

 double axis is the result of dichotomy. The observations 

 of Lereboullet on the ova of fishes points to the same 

 conclusion. He says : 



" Le bourrelet en effet constitue le veritable germe em- 

 bryonnaire, germe toujours simple, unique comme le vitellus 

 que le blast oder me recouvre, mais susceptible quand le 

 developpement se derange, de vegeter comme la substance 

 dont se compose le corps des polypes, et de produire des 

 formes variees qui montrent toujours une tendance manifeste 



* Vrolik, Allen Thomson, and others have placed this matter 

 beyond doubt. 



f M. Coste, Coinptes Rendus, 1855, p. 931. 



