88 THE BIOLOGY OF TWINS 



inhibiting the dominance of the apical end and of 

 producing a whirl of secondary growing points. 



Now in the armadillo egg the ectodermic vesicle has 

 an apical point, which is the head end or growing tip 

 (see Fig. 12, X) of the embryo before the process of 

 fission 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 metaboHsm 

 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 become 

 the first visible growing points, the so-called "primary 

 buds"; those that are less favorably situated acquire 

 independence a little later and form the so-called 

 "seco/idary buds." It happens that, almost synchro- 

 nously 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 

 consequent speed of growth. This rejuvenating factor 

 stops the production 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 individual but is a clone, consisting of four 

 essentially separate individuals, each of which goes 

 through its own embryonic development quite inde- 



