700 



when the embryo is stuck just in front of the dorsal lip i. e. in the black 

 cells of that region. If the material of the dorsal lip grows backwards, 

 then the point of injury should move also and keep its distance from 

 the edge of the lip. If, however, the material in front of the dorsal 

 lip is stationary, then the dorsal lip ought to appear farther and 

 farther from the injury. The latter alternative was found to be true. 



Injuries immediately in front of the dorsal lip of the early blas- 

 topore are found later, as Roux stated, in the cross connective at 

 the anterior end of the medullary plate. Sometimes the injuries are 

 just in front of the anterior connective, sometimes in the connective, 

 and sometimes just behind it. More than twenty-five experiments gave 

 these results. 



In another set of embryos the point of injury was made farther 

 in front of the dorsal lip, i. e. higher up in the black hemisphere. 

 When the embryo formed the defects were found far anterior to the 

 medullary plate, and on the ventro-anterior side of the embryo. 



In a series of thirty embryos the white cells were injured just be- 

 hind the dorsal lip of the early blastopore. The result was the same 

 as when the dorsal lip itself was injured. A large yolk-plug was left 

 in most cases somewhere along the dorsal mid-line; and the medullary 

 folds and notochord were divided into right and left halves on the 

 sides of the yolk-plug. In several embryos where the injury was small, 

 the dorsal lip succeeded in growing over it. 



A series of more than one hundred experiments were made by in- 

 juring the embryo at definite points of the early gastrula stage (and 

 carefully watching the developing egg) to see if any change in posi- 

 tion of the place of injury took place. When the embryo is slightly 

 stuck (so that a small protrusion is formed) at one side of the median 

 (embryonic) line, and anterior to the dorsal lip of the early blastopore, 

 at a time when the latter has just begun to grow over the yolk, we 

 find that the point of injury retains at first very nearly its relative 

 position to the median line, later when the medullary plate is forming 

 the point may shift slightly towards the middle line. 



The larger number of these experiments were made, however, by 

 sticking the embryo at the time when the dorsal lip of the blastopore 

 had just formed (and also at later stages) at one side of the black- 

 white line i. e. behind and to the side of the dorsal lip. The points 

 of injury were well outside of the white and lay in the black cells 

 and to the right or left of the median plane. As the dorsal lip grows 

 over the yolk it is seen to approach nearer to the level of the point 

 (or points) of injury, and then (if the injury were not too far back) 



