106 MR. NOEL TAYLER ON A UNIQUE CASE OF 



described cases of Duplicitas asymmetros in an important and 

 significant respect. Tliat is the fact that the greater embryonal 

 formation (" autosite '"), as well as the lesser, exhibits extreme 

 defects of organisation. It is unnecessary to comment further 

 here upon this interesting fact, for its morphogenetic significance 

 was more or less directly considered in Part III. One may 

 merely emphasize here that it is this characteristic which directly 

 and indirectly affords strong evidence as to the mode of origin 

 of the blastoderm, and further, that while this fact is morpho- 

 genetically of the utmost significance, it indicates no fundamental 

 difference between this specimen and other forms of asymmetrical 

 duplicity as regards their ultimate mode of origin. 



Apart from the blastoderm referred to above, there are a 

 number of cases on record, the significance of which appears far 

 more uncertain ; it would seem difficult even to arrive at a definite 

 conclusion as to whether they can be regarded as monozygotic in 

 origin. Such, for instance, are blastoderms upon which occur two 

 embryos, both of which are morphologically normal, but which 

 are very unequal in their degree of development as indicated by the 

 number of somites. Such forms, it would seem, admit of equally 

 plausible explanation, on either the Dizygotic or Monozygotic 

 hypothesis. In the former case the explanation of the difierence 

 of age of the two rudiments is clear; in the latter they might 

 perhaps be regarded as arising from a merely quantitative unequal- 

 division of the germ, such that each centre received its full 

 complement of " fonnative substance," yet the two centres were 

 quantitatively unlike in such a way as to result in a retarding of 

 the development of one of them. 



If this mode of origin be regarded as likely, we then appear 

 to be able to distinguish three main morphogenetic types of 

 duplicity : 



Type 1. In which the original disturbance of the germ results 

 in the production of both qiiantitatively and qualitatively 

 like daughter centres, giving rise to the equal duplicity, 

 the well-known type of Duplicitas symmetros. 



Type 2. In which the two daughter centres are quantitatively 

 unlike, but qualitatively similar, giving rise to two embryos 

 both morphologically perfect but unequal in their stage of 

 development, 



T3'pe 3. In which the daughter centres are both qualitatively 

 and quantitatively unlike, giving rise to such a form as 

 Blastoderm E, in which either, or even both, embryonal 

 centres exhibit gross abnormalities of structure, apparently 

 always of the nature of a defect of organisation. 



The question of modifications and deformations of a secondary 

 nature is too complex and lengtliy to be dealt with here ; it may, 



