Plate VIII. 



Fig. 57. Transverse section through the injured head-fold of the embryo 

 shown in Fig. 59. The main group of injured cells is at op. X ^^^ 



Fig. 58. Transverse section through the injured region of the eml)ryo 

 shown in Fig. 60. The needle has destroyed a considerable portion of the 

 primitive streak material, and has also disturbed the underlying entoderm. 

 The folding of the lateral poitions of the entoderm is an artifact, x 95. 



Fig. 59. This embryo was operated on thirty-four and one-third hours 

 after fertilization, and then incubated for thirty-four hours. The injury 

 ,\^riS made in . the center of the dorsal lip at a distance from the posterior 

 margin equal to about half the width of the needle (see Fig. XVI, a). Pos- 

 terior to the head fold the embryo is normal in every way. x 21. 



Fig. go. The operation was performed on a freshly laid egg, w^hich was 

 then incubated for twenty-six and three-fourth hours. The injury was 

 made on the boundary between the areas opaca and pellucida, in line with 

 the axis of the future embryo (see Fix. XVI, l)). There are twenty-five 

 sections posterior to the injury that show a characteristic primitive streak 

 structure. X 21. 



Fig. 61. Transverse section through the injured region of the embryo 

 shown in Fig. 68, PI. X. Just one-half of the embryo has been affected by 

 injury, x 95. 



