NATURE, SCOPE, AND CAUSES OF TWINNING 7 



prevailing theory that double monsters are products of 

 the fusion of two originally complete and separate 

 embryos. This question is quite crucial for our general 

 interpretation of the nature and causes of twinning. 

 It is a time-honored question and has been discussed 

 pro and con by leaders in the history of zoology for nearly 

 two hundred years. Important names such as those 

 of Caspar Frederick Wolff, Meckel, the two Saint- 

 Hillaires, Knoch, Dareste, Rauber, Panum, and others 

 are associated with the earlier phases of the problem. 

 Within recent times we find such men as Windle, Gem- 

 mill, Kaestner, Wilder, Stockard, and others still taking 

 sides on the vexed question of whether double monsters 

 are derived from the division of one or the fusion of two 

 embryonic axes. Gemmill and Stockard, among recent 

 writers, stand for the fusion of separate embryos; 

 Wilder and myself, on the other hand, hold to the view 

 that double monsters are incompletely divided single 

 embryos. All of these views will be discussed in detail 

 because, to my mind, an acceptable theory of the 

 morphology and the physiology of twinning depends on 

 a correct interpretation of the mode of origin of double 

 monsters. 



If double monsters are merely the incompletely 

 separated and regenerated bilateral halves of an originally 

 single embryo, we have a rational interpretation of situs 

 inversus viscerum and mirror-image symmetry. From 

 this point of view we can proceed to an understanding 

 of various kinds of partial twinning and of such allied 

 phenomena as symmetry reversal in various asym- 

 metrical forms such as echinoderm larvae. The concep- 

 tion that double monsters are products of a partial 



