NATURE, SCOPE, AND CAUSES OF TWINNING 9 



individuals. The second situation, which is technically 

 called hemihypertrophy, or a unilateral gigantism of 

 one half of the body, strongly suggests partial physio- 

 logical isolation of the bilateral primordia, or minimal 

 twinning. The consideration of double monsters of all 

 classes has brought to light the existence of the phe- 

 nomenon known as situs inversus viscerum and other sorts 

 of mirror-image symmetry between the two components. 

 This phenomenon strikes deep at the roots of the physi- 

 ology of symmetry and asymmetry in the vertebrates; 

 hence a chapter is devoted to an analysis of the observed 

 conditions. After the subject of twinning of whole 

 organisms has been concluded the discussion shifts to 

 twinning in tails and limbs. Duplicity in tails turns 

 out to be merely a phase of bilateral twinning involv- 

 ing a posterior growing region, the tail-bud. Twinning 

 in appendages, however, seems to involve certain com- 

 plexities that are not present in bilateral twinning. An 

 understanding of the sources of complexity, however, 

 seems to indicate that the same fundamental causes 

 and consequences of twinning hold good for twinning 

 or duplicity in limbs as for twinning of the whole body. 

 There occur, moreover, comparable phenomena of sym- 

 metry reversal and mirror-imaging that seem to aid in 

 an analysis of the general laws of symmetry. 



In a final chapter twinning as a mode of reproduction 

 is discussed. It is shown that twinning is a form of 

 axiate reproduction, that twinning is not a reminiscence 

 of a lost ancestral asexual phase in an ideal alternation 

 of generations, and that twinning and polyembryony 

 are two entirely different phenomena and should never 

 be confused. 



