110 J. Thos. Patterson. 



(Fig. 57). The result of such an operation leaves but two alterna- 

 tives with reference to the position of the embryonic primordium at 

 the time when the injury was made. Either we must suppose that 

 this primordium was situated in the exceedingly small space between 

 the operated region and the posterior margin (Fig. XVI, a), or that 

 its right and left halves lay along the lateral margins, and were 

 gradually brought together by concrescence. That the latter alterna- 

 tive is the correct one will become obvious from the results of the 

 following experiments. 



Experiment IV. 

 In this experiment the operation was made ten degrees to the 

 right of the median line, the needle being set so that the outer edge 



Fig. XVI. Scheme for operating in Experiments III, IV, and V. 



of the resulting injury coincided with the margin of the blastoderm 

 (Fig. XVI, &). After thirty-six and three-fourths hours of incu- 

 bation the injury was found on the right neural fold in the mid-brain 

 region (Fig. 63). Although the left neural fold is slightly dis- 

 torted, yet the section shows with great clearness that the affected 

 cells are found only on the right side (Fig. 62), All the structures 

 characteristic of this region in a normal embryo, are here found well 

 developed. As we should expect, the mesoderm and chorda are unin- 

 jured, for when the operation was performed these structures were 

 not yet present in the head region. 



Experiment Y. 

 Passing now to the experiment in which the operation was made 

 forty-five degrees to the right of the middle line (Fig. XVI, c), we 



