STRUCTUEE AND DEVELOPMENTAL RATE 185 



when two embryonic shields arise from the germ-ring they occupy 

 positions about 180° apart, or are opposite one another on the 

 yolk-sphere, in nearly 10 per cent of the cases. 



A further question bearing on the relative position of the em- 

 bryonic shields on the germ-ring suggests itself. If the germ- 

 ring is actually a potentially budding stock, why does not trip- 

 let and quadruplet formations appear almost as frequently as the 

 double or twin condition? This question can at least be an- 

 swered with a probable explanation, if not with a completely 

 satisfactory one. In the first place, the extreme tendency of 

 the eggs to form only a single rather than a double individual is 

 important in this connection. There is certainly only a slight 

 chance to form double specimens. The single bud is almost 

 always capable of suppressing further expressions in the blasto- 

 derm. When this capacity is in any way lowered and a second 

 bud arises, the stock is then still further dominated or pre- 

 empted by the presence of the two, and the chance for still a 

 third embryo formation is decidedly reduced. Yet among a 

 hundred multiple cases one triplet may be found. 



Gemmill found one case of three embryos in a trout egg as 

 against over seventy doubles. This specimen had one almost 

 perfect embryo and the other two were very abnormal and poorly 

 developed. Among something less than 150 double fish embryos 

 seen during the past few years I have observed only one triple 

 specimen. This arose from a Fundulus egg that had been inhib- 

 ited during early development by a weak solution of alcohol. 

 One embryo, almost normal, was on the same blastoderm with a 

 double-headed specimen. 



I have never observed, nor found record of a fish's egg contain- 

 ing more than three embryos. 



The conclusions seems warranted that one point of gastrulation, 

 or embryo formation, has an extremely high tendency to prevent 

 or suppress the existence of any other such point of excessive pro- 

 liferation.' When a second point is capable of expression the two 

 almost without fail completely dominate the growth capacity 

 of the entire germinal region and triplets are the rarest exception. 



