120 BIOLOGICAL LECTURES. 



If one will take the trouble to compare this formation with the 

 ordinary type of teleostean development, he will not fail to see 

 that the organizing forces, whatever they may be, operate to 

 form an embryo under peculiar difficulties. It will be seen 

 towards the end of embryogenesis that the material of the 

 germ-ring, owing to the enormous size of the egg, has to travel 

 over quite a long distance, in order to reach the embryo. A 

 very thin bridge of cells connects the hind end of the embryo 

 with the closing germ-ring, and this bridge is formed by the 

 migrating cells of the germ-ring. What determines this 

 wholly exceptional rnovement of the cell-material required to 

 form the embryo ? Is it possible that the cells move as so 

 many independent individualities ? But they do move, and 

 no doubt in obedience to directing influences, acting, not in 

 the cells as individuals, but in and through the entire forma- 

 tive material, irrespective of cells. 



Whoever doubts this would do well to study more faithfully 

 the living embryo during its formation. If the cell-ghost 

 should still haunt his vision, I would suggest still another field 

 for study. I would suggest first of all that he try to get as 

 clear a notion as possible of the formation of the archenteron 

 in Amphioxus, Petromzyon, and the Frog. The case of the 

 reptile might then be studied with profit. Next the " chorda- 

 canal " of mammals, and finally Kupffer's vesicle in the 

 teleost. 



There is no longer any doubt in my mind and here 

 I am in accord with most authorities on this subject that 

 this little vesicle is a reminiscence of the archenteron. The 

 development of Gecco, as traced by Ludwig Will, removes, as 

 I think, the last doubt on this point. 



If the development of Kupffer's vesicle be studied in the 

 light of its phylogenetic significance, and studied in the living 

 as well as the dead egg, I cannot help thinking that candid 

 reflection on the facts will be sufficient to force conviction to 

 the standpoint here taken. 



Having learned the meaning of the vesicle, one should trace 

 step by step its mode of origin in the pelagic fish-egg. Here 

 one may see this remnant of an archenteric cavity arise, not 



