STRUCTURE AND DEVELOPMENTAL RATE 179 



There can be little doubt from the experiments recorded above 

 and the results to be given below that the environmental condi- 

 tions or external factors are of greater importance than the in- 

 ternal tendencies in twin formation. It is evident that eggs, 

 although capable of producing more than one embryo, rarely ever 

 do. The number of twin formations in a given lot of eggs may 

 experimentally be increased so greatly in excess of the natural 

 occurrence of such individuals, that we are forced to believe 

 even in the cases before mentioned of the armadillo and the exces- 

 sive occurrence of twins in certain families; as cited by Daven- 

 port, that the en\'ironment may in these instances also be the act- 

 ually direct cause. A peculiar uterine reaction may be inherited 

 in the armadillo and in certain human families which prevents a 

 ready or rapid placentation and thus primarily brings about an 

 initial slowing of development. There is much evidence of a 

 slow placental formation and a peculiar uterine condition in the 

 armadillo which will be considered more fully beyond. 



This brief estimate of the internal and external developmental 

 factors concerned in twinning has been given just here in order 

 that the reader may appreciate more fully the very different reac- 

 tions shown by Fundulus and the trout. He may form for him- 

 self some idea as to whether this is due to a difference in morpho- 

 logical pattern of the germ-rings or potential budding regions in 

 the two species or to differences in the physiological reactions 

 to the environment or finally to a combination of both. 



Double and twin trout are classical objects, they often occur 

 in the hatcheries in various parts of the world and have been 

 frequently figured and described since the early studies of Lere- 

 boullet by Rauber, de Quatrefages, Klaussner, Gemmill, and 

 others. 



All of the double individuals and twins recorded have been 

 surprisingly well developed and normally formed. From figures 

 and descriptions, it would seem as though the trout egg possessed 

 a rather normal tendency to form double embryos, and the causes 

 necessary to give expression to this tendency were so slight as 

 not to be further injurious to the development of the individual 



