Gastrulatiou in the Pigeon's Egg. 113 



yet they are capable of giving rise to the normal structures character- 

 istic of this region. Posterior to the injury, however, they have suc- 

 ceeded in fusing and forming the primitive streak material. 



Experiment VII. 

 If an operation he made similar to the preceding, but at a slightly 

 later period, it is not found in the body of the embryo, but at the 

 extreme posterior end (Fig. 64, op). 



Experiment VII I. 

 In this experiment the injury was made twenty degrees to the right 

 of the axial line (Fig. XVIII, &) at thirty-six hours after fertiliza- 

 tion, that i§, at a stage corresponding to the one shown in Fig. V. J. 

 The egg was then incubated for forty-eight hours. The injury is 



Fig. XIX. Ti-ansverse section through the injured region of the embryo 

 shown in Fig 54. X 95. 



located on the right neural fold, at about one-third the distance from 

 the posterior end to the first mesoblastic somite (Fig. XX, op). The 

 left neural fold is uninjured. The result of such an experiment 

 admits certainly of no other explanation than that the mass of 

 affected cells has moved from a marginal into an axial position. 



Set C. On Unincubated and Early Incubated Stages. 



In this set of experiments I shall endeavor to show that not all 

 of the embryonic material has been brought into a median or axial 

 position at the time when the egg is laid ; but that it lies to either 

 side, on the boundary between the areas opaca and pellucida. The 

 presence of a slightly more opaque spot in this region has already 

 been noted in connection with the study of surface views (Fig. V,/^), 



