62 THE PHYSIOLOGY OF TWINNING 



Starting with Gemmiirs idea that in fishes the 

 germ ring may be regarded ''as a stock, able to give rise 

 vegetatively, so to speak, to more than one embryo," 

 Stockard builds up a theory of twinning which I may 

 call the ''accessory budding theory." The analogies 

 are drawn largely from the plant world. In Bryo- 

 phyllum, for example, it is known that the notches 

 around the border of the leaf have the power to bud 

 and give rise to new plants. As a rule, under ordi- 

 nary atmospheric conditions, only one or two notches 

 give off new shoots. The presence of these shoots seems 

 to inhibit the appearance of others. If, however, these 

 other notches are isolated, each may produce a new 

 shoot. 



The periphery of the blastoderm in the eggs of the bird and 

 mammal or the germ ring in a teleost's eggs is probably in some 



sense comparable to the notched border of the budding leaf 



There are many potential points around the germ ring at which 

 an embryonic axis might arise. Here again, as in the plant, 

 when one bud or embryonic axis has arisen, it tends to suppress 

 the potential ability of other points to form an axis, and normally 

 only one individual is developed in the egg. 



We are entirely unable to state the reasons why a certain 

 point along the genn ring should form the bud and not another. 

 One can only imagine that this point has some pecuUar advantage 

 of position which gives to it a higher power of oxidation and a 

 temporarily more rapid rate of cell proHferation than is possessed by 

 other points, just as the notch which is dipped below the water sur- 

 face possesses a budding advantage over the other notches around 

 the leaf. Can the advantage of position possessed by a particular 

 point of the germ ring be reduced so as to equalize the budding 

 tendency of several points and thus allow them to express their 

 ability to form embryonic axes? Could such a condition be 

 brought about, double embryos, twins, triplets, etc., would be 

 produced. 



