m 



sticking with a needle definite portions of the frog's embryo at the 

 gastrula stage. These results of Roux are of great importance, for 

 if true, they show the method by which we must regard the blastopore 

 to closed. Further; "Roux's experiments, by sticking the border 

 between black and white regions, point directly to a process of con- 

 crescence of some sort. If Roux's experiments are accurate, we must 

 suppose that the cells that will later form the central nervous system 

 are already laid down along the black-white border. These cells must 

 come up to the middle line as the blastopore gets smaller. The clo- 

 sure of the blastopore from before backwards would then be due, not 

 to a backward extension of all of the material of the dorsal lip over 

 the yolk, but would take place by new tissue coming up to the middle 

 line from the sides and placing itself with or behind cells already 

 present in the dorsal lip". In a subsequent paper (6), I added in 

 regard to Roux's experiments, "one must either doubt the validity of 

 these, or their interpretation. The experiments need careful repeti- 

 tion", for at this time I was inclined to believe that both in the fish 

 and frog the dorsal lip of the blastopore grew backwards in the middle 

 line during its overgrowth of the yolk. I attempted to explain 

 those abnormal embryos with dorsal exposure of yolk (spina bifida) as 

 due to a division in the posterior extension of embryonic tissue (i. e. 

 a division of the dorsal lip of the blastopore), and I believed that 

 these divided portions subsequently united, around the obstacle into 

 a single structure again median in position *). 



During the present spring ('94) I have made a large number of 

 experiments to test Roux's conclusions, and I fully acknowledge that 

 in this case my scepticism was unfounded, as the results below 

 will show. In a large number of eggs the dorsal lip was stuck with 

 a sharp, cold needle at the time of its first appearance. A pro- 

 trusion of material followed, sometimes pushing through the membrane, 

 and at other times remaining between egg and membrane. The dorsal 

 lip was completely destroyed, nevertheless, the lateral lips of the bla- 

 stopore appeared, and even the posterior lip. The time of appearance 

 of these parts was nearly the same as in the normal embryos. The 

 closure of the blastopore followed, but owing to the injury to the dorsal 

 lip, the closure was modified. A V-shaped outline at first formed with 

 the apex of the V at the dorsal lip, and later an elongated opening 

 remained, the outline varying in different eggs. There is generally 



1) An unfortunate mistake of the printer makes the sentence quoted 

 appear meaningless in the original. It has been set right above. 



