376 



Florence Peebles. 



Experiment III. In another set of experiments the two injuries 

 were made about .5 mm. further forward (Text-fig. 9, x and 0). 

 The position of the injuries after forty hours is seen in PI. I, Fig. 

 7. The wound at the side (0) has advanced with the growth of the 

 blastoderm but the wound (x) in front of the somites has prevented 

 the formation of the head, and the embryo is reduced in length 

 anteriorly, the trunk and caudal regions are about the normal 

 length. 



From these experiments it seems evident that the region in front 

 of the middle point of the area pellucida is the seat of active 

 growth in an anterior direction. 



Experiment IV. In order to determine the extent of growth 

 posteriorly, two injuries were made, one in the center of the unin- 

 cubated blastoderm and the other in its posterior margin (Text- 

 fig. 3, X and y). The embryos were incubated thirty-six hours to 



two days. They developed somites and medullary folds in the 

 area between the wounds. PI. II, Fig. 8, represents a surface view 

 at the end of thirty-six hours. Notochord and somites have 

 developed between the two wounds. The actual distance from 

 X to y before the experiment was i mm. After incubation it was 

 3 mm. showing an increase in length of only 2 mm. The normal 

 embryos at this age measure 4 mm. from heart to caudal end. 



The results of these three sets of experiments show that the 

 embryo may be greatly reduced in length by preventing growth 

 anteriorly with the wound x and posteriorly with the wound y^ 

 and that the area pellucida grows less rapidly at the sides than in 

 the median line. 



Up to this time the experiments which I have described have 

 been made upon the unincubated blastoderm. The change in 

 the size and the shape of the area pellucida is comparatively 



