Gastrulation in the Pigeon's Egg. 115 



the boundary between the two areas. If from the former source, 

 the material would be disturbed by an injury made in the area 

 opaca just posterior to the pellucid area ; but if from the latter source, 

 it would be affected only by operations made to the side of the axial 

 line on the boundary between the two areas. 



Experiment X. 

 The operation was made just posterior to the pellucid area (Fig. 

 XXI,a). The injury has in no way affected the embryo (Fig. 56), 

 but lies posterior to it in the. area opaca. Assheton ('96) has per- 

 formed a similar experiment on the chick blastoderm, using a sable 

 hair instead of the needle. lie also found the embryo uninjured. 

 From the result of tliis operation it is evident that the material 

 out of which the tail of the eml)ryo is differentiated does not lie 

 just posterior to the pellucid area. 



Fig. XXI. Scheme for operating in Experiments IX-XIII. 



Experiment XI. 

 If the injury be made on the boundary between the two areas 

 twenty degrees to the right of the middle line (Fig. XXI,c), it is 

 found later drawn into the side of the embryo (Fig. 68) a short 

 distance from the posterior end. In the transverse section through 

 the operated region, it is seen that just half of the axial material is 

 affected (Fig. 61). This is of the greatest importance, because it 

 shows that the tail end of the embryo is formed by the concrescence 

 of material lying to either side of the middle line. The limit to 

 which material extends laterally is shown in the following ex- 

 periments. 



