AMPHIBIA, REPTILES, OTHER CHORDATES 97 



costal plates and accessory costal scutes. In view of 

 what we now know about twinning in vertebrates, I am 

 convinced that this strong tendency to form a double 

 median series of scutes and plates in these subnormal 

 turtle embryos is a case of incipient twinning, due to 

 partial isolation of the median dorsal elements of the 

 right- and left-hand primordia of the axis. That the 

 twinning process sometimes goes much farther than this 

 is evidenced by the fact that two-headed conjoined 

 twins, such as that shown in Figure 45, actually occur. 



Why, we may ask, does not twinning occur more 

 frequently when the environmental conditions appear to 

 be such as to favor it ? It seems to me highly probable 

 that the same reason appHes here as in the birds: that 

 the eggs have passed the critical period before they are 

 laid. There is available some direct evidence that the 

 chelonian embryo is a little farther along than is that of 

 the bird at the time of laying. Consequently we might 

 expect only minimal twinning to occur, viz., that in 

 which the already established axis of symmetry is affected 

 so as to bring about a more or less extensive fission of 

 the bilateral primordia, especially those in the median 

 dorsal position. If then scute and plate irregularities 

 are to be interpreted as the result of a minimal phase of 

 twinning, my earher interpretation of these ''abnormali- 

 ties" as reversions to an ancestral condition (Newman, 

 1906) will have to be modified, and I shall have to confess 

 that at one time I was less cautious than I now would 

 be as to the interpretation of anomalous or abnormal 

 biological materials as evidence of phylogenetic or ances- 

 tral conditions. Most of us have at some time or other 

 fallen into this familiar type of error. 



