DOUBLE MONSTERS IN FISHES 63 



The chief criticisms of this point of view are these: 



a) The germ ring is not a stock from which buds 

 appear, for, long before there is any germ ring the axis 

 of the single or of twin embryos is already established. 

 The situation in the bony fish is similar to that in the 

 starfish. The embryonic axis is established at an early 

 blastoderm stage, as has been shown by the analyses of 

 Dr. Hyman (192 1). One can readily determine that 

 one side of the blastoderm is the posterior end, or the 

 end at which gastrulation will occur. There is no such 

 thing as a germ ring until long after the blastoderm 

 exhibits definite axiate relations. When two embryonic 

 shields arise they have originated through a breaking 

 down of the original polarity and a physiological isolation 

 of two regions of gastrulation on the blastoderm before 

 any germ ring is present. When the germ ring appears 

 it is merely a non-embryonic region of the blastoderm, 

 concerned primarily with yolk overgrowth, and it ulti- 

 mately forms merely the neck of the yolk stalk. 



b) If the early blastoderm, long before germ-ring 

 formation, already has a definite anterio-posterior axis 

 and the point of gastrulation is well established, we can 

 hardly agree with Stockard's assertion that ''we are 

 entirely unable to state the reasons why a certain point 

 along the germ ring should form the bud and not 

 another." Surely the location of the region of gastrula- 

 tion is not due merely to ''some peculiar advantage of 

 position which gives it a higher power of oxidation and 

 a temporarily more rapid rate of cell proliferation." 

 Gastrulation is the same process wherever it occurs, 

 and it takes place at points predetermined by the original 

 polarity of the egg. One only need refer to the case 



