STRUCTURE AND DEVELOPMENTAL RATE 177 



Certain delicate or sensitive eggs will probably respond more 

 readily and give double conditions more frequently than hardier 

 eggs. The eggs of Fundulus are very hardy, and it may be that 

 a treatment when acting in a delicate manner affects favorably for 

 our purpose only the more sensitive eggs, while the large majority 

 are too resistant to respond. Should the conditions be more severe 

 they would act too harshly to obtain a double response from any. 

 These speculations will appear to have a stronger foundation 

 after we have reviewed the very remarkable tendencies on the 

 part of the delicate eggs of the trout to give double and twin 

 embryos. 



2. Double embryos in trout eggs. The eggs of the trout un- 

 questionably possess a stronger innate tendency to form double 

 and twin individuals than do those of Fundulus. Twinning and 

 double formations, like all other unusual developmental phenomena, 

 are not simply and entirely due to the action of an ususual environ- 

 ment, but also depend upon the internal structure of the given egg 

 and its peculiar manner of development. An environmental stim- 

 ulus which would frequently induce double formations in one 

 type or species of eggs might be completely ineffective in its 

 action on the eggs of another species. The burden of evidence 

 for the cause of twin formation as well as the means of artificially 

 inducing it indicate an accessory budding or double blastopore 

 formation as the primary step, and it is obvious that the early 

 morphology of certain eggs more readily lends itself to the estab- 

 lishment of accessory blastopore formations than does that of 

 others. 



Not only is this morphological difference to be expected, but 

 from what we know of the physiology of budding, it is also 

 logically probable that in certain eggs the bud for the embry- 

 onic axis will arise with higher powers for dominating the entire 

 budding region than in others. Different degrees of dominance 

 of the apical bud in different plants is a well-recognized fact. 

 In some plants the terminal bud grows to form a long slender 

 stalk, without producing axillary or lateral shoots, while the 

 terminal shoot of other plants grows to a limited extent only 

 before axillary buds and branches make their appearance. The 



THE AMERICAN JOURNAL OF ANATOMY, VOL. 28, NO. 2 



