178 CHARLES R. STOCKARD 



embryonic axis in the vertebrate egg may be compared with the 

 terminal shoot of a plant. A second embryonic axis formation is 

 roughly comparable to the occurrence of an ordinary lateral or 

 axillary plant bud. A better comparison, however, is made be- 

 tween the animal egg and the budding leaf, such as Bryophyllum. 

 In both of these, the egg and leaf, we may recognize an area of 

 potential budding capacity, and we know that not only one, but 

 several equal buds may arise simultaneously from either of these 

 two stocks. There are doubtless certain kinds of budding leaves 

 which are more prone to form multiple buds than others. From 

 such leaves instead of one shoot arising in a certain notch, several 

 notches are equally capable of budding and several shoots are 

 formed. It would seem that certain animal eggs also normally 

 possess a disposition to produce several buds. Such eggs develop 

 more than one embryonic axis and give rise to several individu- 

 als instead of the usual single embryo from a single egg. This is 

 probably the case in the Texas armadillo. 



The eggs of Fundulus and those of the trout, very probably 

 illustrate two different degrees of capacity to form several in- 

 stead of one embryonic bud. This much of the twinning pro- 

 cess is truly inherited, and variations in the tendency may occur 

 not only among the eggs of different species, but probably also 

 exist among the individual eggs of the same species. For ex- 

 ample, certain mothers may produce eggs highly inclined to give 

 rise to two embryonic buds or twins, and such an inclination may 

 be transmitted or inherited by her daughters or even through her 

 sons. Davenport ('20) has recently found in a study of human 

 twins that the males of a family carry or transmit the twinning 

 tendency in equally as evident a manner as do the females. Double 

 and twin individuals are also of much higher frequency in certain 

 human families than in the community as a whole. All this indi- 

 cates that the eggs of certain individuals are more inclined to 

 form twins than are those from others. In the human cases 

 the present consideration refers, of course, only to so-called iden- 

 tical and not fraternal twins. The latter, truly speaking, are 

 not actually twins. 



