322 H. H. NEWMAN 



developmental condition. Seven years ago the writer published 

 a theory purporting to explain on a physiological basis the 

 extraordinary phenomenon of specific polyembryony in the nine- 

 banded armadillo (Dasypus novemcinctus) . The tenor of this 

 theory was that the armadillo egg became temporarily retarded 

 so as to cause a slowing down of its metabolic rate or intensity 

 and a consequent loss of its axiate structure; that when devel- 

 opment was resumed four centers of differentiation arose instead 

 of the original one, and quadruplet embryos were the result. 

 Subsequently Patterson discovered that the armadillo blasto- 

 dermic vesicle, in a stage equivalent to a gastrula, lies quiescent 

 in the fallopian tubes or in the uterine cavity for a period of 

 perhaps three months. On the basis of this observation, the 

 writer, in his book on The Biology of Twins, formulated the 

 following theory: 



In the armadillo egg the ectodermic vesicle has an apical point, 

 which is the head end or growing tip of the embryo before the process 

 of fission (polyembryony) occurs. If the conditions of growth were 

 such as to admit of a normal rate of metabolism, this original apical 

 end would become the head end of a single embryo. Some agency 

 lowers the rate of metabolism of the embryo and the original apical 

 end loses its dominance over subordinate regions; the result is that 

 several radially arranged secondary points in the ectodermic vesicle 

 acquire independence. Those that are most favorably situated with 

 reference to the uterine axes express their independence first and be- 

 come the first visible growing points, the so-called 'primary buds;' 

 those that are less favorably situated acquire independence later and 

 form the so-called 'secondary buds.' It happens that, almost syn- 

 chronously with the physiological isolation of the whirl of subordinate 

 growing points, a new and effective nutritive connection (the Trager 

 ring) is established between the embryonic vesicle and the maternal 

 tissues, which greatly accelerates the metabolic rate and the conse- 

 quent speed of growth. This rejuvenating factor stops the produc- 

 tion of further growing points and makes it possible for each of the newly 

 formed apical ends (heads) to develop a body. When the conditions 

 of growth are restored to normal, the vesicle is no longer a single in- 

 dividual, but is a clone, consisting of four essentially separate indi- 

 viduals, each of which goes through its own embryonic development 

 quite independently of the others, except in so far as development 

 within a common chorion and the necessity of sharing a single pri- 

 mary placenta involve mutual adjustments. 



