2O2 H. H. NEWMAN. 



hardly fix itself so that its dorso-ventral axis would correspond 

 with that of the uterus, so we are driven to the conclusion that 

 the embryo at the time of fixation has no bilaterality, but acquires 

 its bilaterality from that of the mother. The first indication of 

 bilaterality is seen when the vesicle elongates toward the right 

 and left oviducal openings of the uterus and when bilateral buds 

 of ectoderm grow out to the right and left sides. In other words 

 the bilaterally symmetrical arrangement of the embryonic pri- 

 mordia is imposed upon the vesicle by the form of the uterus. 



These first embryonic buds, the primary buds, stand for two 

 pairs of twins, each bud redividing to form two secondary buds, 

 each of the latter going to form a separate embryo. The closer 

 resemblance between the twins of one side is simply the expected 

 result of their community of origin, for they are the product of 

 the longitudinal splitting of a single primary bud of ectoderm. 

 In other words both have been derived from a rather limited 

 area of cells which is separated from the primordia of the opposite 

 pair by a considerable zone of extra-embryonic tissue. On any 

 theory of unequal distribution during cleavage of differentiating 

 factors we would expect these twin products of the horizontal 

 splitting of a common primordium to be closely similar to each 

 other and to show a relatively large amount of mirror-imaging. 



(c) Bateson' s Views Concerning Twinning. In his book 

 " Problems of Genetics" Bateson has an interesting and suggestive 

 chapter on "meristic phenomena" in which he lays great stress 

 upon the fundamental importance of the problem of cell division 

 and other division phenomena. He conceives of the phenomena 

 of bodily symmetry as a direct result of the symmetrical features 

 of cell division. Twinning in mammals is for him a special case 

 of the effects of cell division for he adopts the traditional view that 

 the twins result from a physiological isolation of the blastomeres 

 of the two-cell stage of cleavage. Bateson accepts Wilder's 

 position that duplicate twins are in a series with double monsters 

 of various grades. Double monsters show all degrees of mirror- 

 imaging, while duplicate twins show only vestiges of this phe- 

 nomenon. Much emphasis is laid upon the few cases of reversed 

 finger-print patterns in the duplicate twins described by Wilder. 

 Any sort of mirror-imaging (symmetry reversal) is an indication 



