701 



to pass beyond that level. At first the injured region remains sta- 

 tionary, but as soon as the outline of the lateral lips of the blastopore 

 is well formed, then a decided movement towards the median line 

 begins. I have seen the point of injury pass through about sixty de- 

 grees. Ultimately the defect is often found in the side of the medul- 

 lary plate. The best results have followed from those cases where 

 the protrusion resulting from the injury was slight and did not break 

 through the egg membrane. 



There are distinct traces of a rotation of the egg as a whole 

 during this period, so that when the injured portion protruded through 

 the membrane to form an 'extra-ovate', this was often left behind as 

 the egg rotated, and a fine line of whitish cells was left over the sur- 

 face of the egg extending from the extra-ovate in the jelly to the 

 point on the egg previously injured. Any conclusion therefore drawn 

 from the position of the extra-ovate in the membrane would certainly 

 lead to error. 



Another series of experiments was made which gave more posi- 

 tive evidence against any secondary bifurcation of material in the 

 dorsal lip. If we injure the embryo at two points on the same side 

 along the black-white line, the first injury immediatly posterior to the 

 blastopore (that has just formed) and the second farther back than 

 this, we shall delay or prevent the possible extension from the 

 dorsal lip of any material along the black-white line. Did this exten- 

 sion really take place it would be delayed by the first injury, and if 

 this were passed, it would again be delayed by the second. Moreover, 

 since both points of injury are well off to one side, nothing would 

 prevent the dorsal lip from extending directly backwards, were this 

 its method of growth. 



On the other hand, if the material is already laid down in a 

 ring around the black- white line, then there is no reason why the 

 blastoporic outline should not appear between the two points of injury 

 and posterior to the more posterior injury, at the same time that it 

 forms on the other uninjured side of the blastopore. 



The embryos show that the blastopore forms at nearly the same 

 time on each side, although the progress to the middle line is some- 

 what delayed on the injured side. The dorsal lip shows no tendency 

 to move independently over the yolk, but its progress depends upon 

 the success which the material on the injured side has in its move- 

 ment towards the middle line. The experiment shows in the most 

 satisfactory way that the material comes up from the sides, and that 

 the dorsal lip itself does not extend from before backwards. We 



