ASYMMETRICAL DUPLICITY IN THE CHICK, 105 



2. That the two embryonal centres were, from the earliest 



period, dissimilar. 



3. That the dissimilar potentiality of the two centres almost 



certainly resulted from their mode of origin, i. e., from a 

 qualitatively unequal division of the " formative substance " 

 of the original germ. 



Here, then, we have at least one well-defined morphogenetic 

 type of asymmetrical duplicity, that which owes its essential 

 characteristics to the production of two daughter centres of unequal 

 potential from, in the first place, a single and presumably normal 

 centre. It seems impossible that the two embryonal rudiments 

 on this blastoderm can ever, to adopt Wilder's terminology, have 

 been " physical duplicates." 



It is not possible in this paper to enter into a comparative 

 discussion of unequal monstrosity, even in the veiy restricted 

 field of avian teratology. In few cases have instances of unequal 

 monstrosity in birds received sufficiently detailed description to 

 make this a fruitful undertaking, however desirable. But at 

 least this may be said : — 



That there are at least one or two cases of unequal mon- 

 strosity which must have resembled Blastoderm E in their mode 

 of origin so closely as to belong to the same morphogenetic type. 

 Such would appear to be — 



(a) A goose blastoderm described by Grundman (5) and quoted 



by Schwalbe (16). 



(b) Three chick embryos described by Dareste (2) and also 



quoted by Schwalbe. 



(c) Possibly a blastoderm described by Kaestner (6) consisting 



of a normal chick embryo of 1 7 (?) somites, latercdly to 

 which occurred a primitive streak-like rudiment, closely 

 resembling, according to his figui'e. Rudiment /3 of 

 Blastoderm E. There was apparently no sign of any 

 difterentiated morphological connection between the two 

 rudiments. 



The blastoderms described by Grundman and Dareste resemble 

 each other and differ from Blastoderm E in that the lesser 

 embryonal rudiment has attained a considerably higher degree of 

 development than is the case in the latter. 



In all these cases the lesser rudiment is characterised by the 

 extreme deformation and lack of development of the head and 

 by the absence of the heart. In none of these cases is there any 

 description of sections. Fortunately, however, I happen to have 

 in my possession a blastoderm which appears to resemble these 

 closely, and as it appears not devoid of interest, 1 hope shoi"tly to 

 have the opportunity of describing its structui-e. 



It may be noted here that Blastoderm E appears unique among 



