212 THE PHYSIOLOGY OF TWINNING 



rise to several independent embryos, is not twinning. 

 Twinning is essentially a dichotomy, a division of one 

 primordium into two. Repeated dichotomies may give 

 rise to numerous offspring from a single blastoderm as in 

 the South American armadillo, Dasypus hyhridus; or there 

 may be only two dichotomies as in Dasypus novemcinctus, 

 or there may be one complete dichotomy, followed by a 

 partial dichotomy of one twin, as in certain cases of 

 so-called triple monstrosities in fish. Twinning is essen- 

 tially a physiological isolation of two equivalent growing- 

 points due to a partial loss of integration of the bilateral 

 halves of the blastoderm. The completeness or incom- 

 pleteness of the process depends upon the degree to which 

 the dominance of the original apical end has been sup- 

 pressed. Twinning differs primarily from both trans- 

 verse fission and lateral budding in that it is an affair 

 of the axis of symmetry, instead of the axis of polarity. 



II. NON-AXIATE REPRODUCTION 



This type of reproduction has apparently no direct 

 reference to the axiate relations of the parent organism. 

 Small masses of tissues or single cells are isolated in 

 various ways from the parent tissues— usually internal 

 tissues— and are capable under the proper environmental 

 conditions of reproducing new organisms like the parent. 

 It is probably true that the tissues that give rise to 

 gemmules, statoblasts, spores, or gametes, are so related 

 to the axes of the parents that physiological isolation 

 is favored. In that sense we might consider all repro- 

 duction as, in last analysis, axiate, but for our purposes 

 the axiate relation is so vague, if present at all, that we 

 are justified in ignoring its existence. 



