454 MORGAN. [Vol. X. 



ring did not, perhaps, contribute at all (or to any great degree, 

 as I then stated) to the formation of the embryo. Further work 

 has however convinced me that, to some extent, the germ-ring 

 does take part in the formation of the growing embryo, and 

 such results are, I think, in full harmony with the results of 

 both of our experiments. Since only a small part of the 

 posterior elongation is due to material from the germ-ring, and 

 the greater part to the elongation of the tail-knob, we should 

 still expect to find, when the germ-ring is cut off, that suffi- 

 cient material remained in the tail-knob to form nearly a per- 

 fect embryo. My own view also explains the results obtained 

 by destroying the tail-knob. In such cases the major part of the 

 embryo is injured, and its backward growth prevented, hence, 

 since the germ-ring remains still attached to this fixed posterior 

 end, we can understand why the material of the germ-ring does 

 not continue to pass into the embryo. I had performed this 

 same experiment in Teleosts, but with varying results. In 

 some cases the embryo did not grow any farther. In other 

 cases the embryo healed its injury and continued to extend 

 posteriorly. 



Roux, in 1888 (13), described the result of some experiments 

 on frogs' eggs. Definite portions of blastulae were injured 

 with a needle, and the results recorded. At the blastula stage 

 injuries in the first foundation of the gastrula lip gave defects 

 in the cross-commissures of the brain. Injury on the equator 

 gave defects nearly in the middle of the medullary folds. 

 Injury in the middle of the white pole produced no defect 

 in the embryo, but if eccentric, a more or less imperfect union 

 of the medullary folds was produced (Asyntaxia medullaris). 



From these experiments Roux concluded : that the medul- 

 lary folds form, in the frog, over the white pole : that the 

 dorsal lip of the blastopore grows over the white pole through 

 about 1 70° : and that this is brought about by the apposition of 

 the two sides of the blastopore. ^ 



After a statement of His' concrescence theory Roux con- 

 cludes : " Diese Angabe ijber die Bildung eines Knochen- 



1 Die Gastrulation des Froscheies voUzieht sich also wesentlich durch Ueber- 

 wachsung des Aequators aus, also durch " bilaterale Epibole." 



