378 



Florence Peebles. 



growth of the embryo, was left on one side. The only disturbance 

 evident was in the medullary folds and somites on the side of the 

 injury. 



Experiment VI. A wound at the posterior end of the primitive 

 streak is alone sufficient to shorten the embryo caudad. In PI. II, 

 Fig. II, a surface view of an embryo of forty hours' incubation is 

 shown. The wound (y) was made at the posterior end of the 

 primitive streak of eighteen hours. Fourteen pairs of somites are 

 present, and the embryo measures 3 mm. from the heart to the 

 anterior border of the brain. This region is normal, but growth 

 in a posterior direction has been stopped, by the wound, and the 

 length is reduced 1.5 mm. 



Summary. The results from these experiments show that in 

 the formation of the third-day chick neither head nor tail region 

 can be taken as fixed points, indeed no one point on the blastoderm 

 can be said to be fixed. In the series of diagrams (Text-fig. 11, 

 A-G) I have indicated, in a schematic way, the method of growth 

 from the beginning of incubation until the third day. The growth 

 of the area opaca is symmetrical therefore it is not included in the 

 diagram. The line I-J represents the plane dividing the unin- 

 cubated blastoderm into anterior and posterior halves, and passes 

 through the region in the older embryos which corresponds to the 

 middle point of the area pellucida before incubation. From this 

 point growth proceeds in all directions in the plane of the blasto- 

 derm. The growth from the first to the twelfth hour is sym- 



