THE LnriTS OF TTEREDTTY 179 



the two- or four-cell stage, or the formation of double, 

 triple, or multiple monsters in the blastula, gastrula, 

 or larval (bipinnaria) stage. 



It is of interest to note in this connection that 

 pol^^embryon}^, through division of the proembr^-o 

 into four cells which undergo separate development, 

 is a common feature in the pines and in some other 

 Abietineae, although in Pinus laricio it has been 

 observed that a single embryo sometimes develops 

 from these four cells owing to their failure to separate. 

 In an}^ case only one embryo reaches maturity, the 

 others aborting at various stages of their develop- 

 ment. It is unknown what determines the cohesion 

 or separation of the four proembryonic cells. 



Reversed symmetry and mirror-imaging are the 

 result of a unit}' in development, and the degree in 

 which these phenomena appear in twins furnishes 

 evidence as to the length of time which the embryos 

 remain in contact during development. It is now 

 generally agreed that double monsters are simply 

 conjoined twins which have failed to complete their 

 separation. Wilder (191 6) describes a remarkable 

 case of double twins (girls) of the extremely rare 

 pygopagous* type. Their four palms are practically 

 alike in ridge pattern. Three of the soles are also 

 alike, but the fourth is radically different. Curiously 

 enough, the data concerning the palm and sole 

 patterns of the parents, which are necessary for an 

 evaluation of the inheritance, are not mentioned, and 

 this limits the interpretation of the facts. One cannot, 

 therefore, say that the aberrant pattern is beyond 

 hereditary control. 



The famous Blazek twins, who were born in 

 Bohemia and recently died in Chicago at the age of 

 forty- four, are apparently another case of pygopagy. 

 One of them was married and gave birth to a son, 



* Joined by the buttocks. 



