112 J. Thos. Patterson. 



experiments that similar operations made at a later period of 

 development should be found located more posteriorly in the resulting 

 embryo. Thus in a late gastrnlation stage, an injury made in the 

 middle of the dorsal lip just anterior to the posterior margin, should 

 be found later not in the fore-brain, as in Experiment III, but at a 

 point situated more posteriorly to that region. Furthermore, if 

 an injury be made on the margin to the side of the axial line at 

 such a stage, it ought later to appear in the corresponding side toward 

 the posterior end of the embryo. The results of such operations are 

 shown in the following set of experiments. 



Set B. On Late GasteUlar Stages. 



Experiment VI. 



The scheme for this operation appears in Fig. XVIII, a. It 



would be equivalent to injuring the posterior margin of such a 



blastoderm as that shown in Fig. V, /. The result of the operation 



is shown in Fig. 54, in which the injury is seen to lie at the level 



Fig. XVIII. Scheme for operating in Experiments VI, VII and VIII. 



of the tenth paii; of somites. Posterior to the affected region there 

 is found a normal primitive streak, which has not yet differentiated 

 into the posterior axial portion of the embryo. The transverse sec- 

 tion (Fig. XIX) shows a mass of dead cells lying between the sepa- 

 rated halves of the neural tube, and a few scattered dead cells lying 

 just above the entoderm, which is intact. The notochord is situated 

 on the right side. 



It would seem from this result that although the halves of die 

 dorsal lip have been unable to coalesce in the region of the injury, 



