452 MORGAN. [Vol. X. 



If we do not assume that material from the germ-ring passes 

 into the embryo, we should expect that two embryos, appearing 

 near to one another on the germ-ring, would, during the early 

 extension of the blastoderm over the yolk-sphere, separate 

 farther and farther from one another, till the equator of the 

 Q.gg was reached. Rauber's evidence, which seems to me, as I 

 say, to be valid, points clearly to the tendency of the embryos 

 to grow together, as they grow older. 



We might assume an innate attractive force that tended to 

 draw such embryos together, but embryology has passed the 

 period when such explanations are acceptable. 



Therefore, accepting Rauber's facts, is there an escape from 

 his conclusion } It seems to me there is. In the preceding 

 pages I have attempted to show that there is evidence pointing 

 to the conclusion that material from the germ-ring does pass 

 — perhaps continuously — from the germ-ring into the em- 

 bryo. But I have also tried to show that this material forms 

 only a small part of the elongating embryo, and does not itself 

 give rise, by apposition, to the entire right and left sides of the 

 embryo. 



Therefore, since the material of the germ-ring does pass 

 into the embryo, we can accept a part of Rauber's conclusions 

 and thus account for the union of two embryos starting near to 

 one another on the germ-ring. The remainder of Rauber's 

 conclusion, viz., that the facts demonstrate elongation by 

 apposition, or concrescence, does not, I think, follow from his 

 observations, or fit in with what seems to be the normal method 

 of the formation of the fish embryo. 



Riickert, in 1887 (14), working on the Torpedo, makes the 

 following statement: " Sonach kann ich die schon im Jahre 

 1876 von His vertretene Anschauung, nach welcher die axiale 

 Anlage der Haie aus einer Verwachsung der sich einfaltenden 

 Blastodermrander hervorgeht, f iir einen beschrankten (hinteren) 

 Abschnitt des Embryo bestatigen. Dass die ganze Embryonalan- 

 lage, wie His dies will, auf solche Weise entsteht, glaube ich 

 allerdings nicht annehmen zu diirfen." " Vom Standpunkt der 

 Gastraatheorie betrachtet, stellt derselbe in der That einen hoch- 

 wichtigen Akt, namlich die Schliessung des Blastoporus dar." 



