38o 



Florence Peebles. 



Experiment III. In a third series of experiments the middle 

 part of the primitive streak was killed (Text-fig. 14), leaving some 

 of the material in front, and some back of the v^ound. According 

 to Kopsch, in the later embryo the region from the first to the 

 twentieth somites should be lacking. 



Nearly all of the embryos which I operated upon, in this way, 

 were so greatly disturbed by the wound that all development was 

 checked. In PI. II, Fig. 14, a surface view of the body region of 

 one of the embryos which developed further is shown. The brain 

 and heart were normal, therefore they are not included in the 

 figure. Posteriorly the wound {w) stretched apart, but anteriorly 

 medullary folds and ten or twelve pairs of somites are present. 

 This result indicates that at least ten or twelve pairs of the first 

 twenty somites come from the material in the anterior third of 

 the primitive streak. 



IS 13 14 15 



Experiment IV. Finally, the anterior third of the primitive 

 streak was killed (Text-fig. 15). After further incubation the 

 embryo developed a normal brain and heart in front of the wound. 

 The trunk region (without the brain and heart) of one of these 

 embryos is shown in PI. II, Fig. 15. Back of the wound (w) 

 eleven to fourteen pairs of somites are present. By comparison 

 with normal embryos of the same age I conclude that these somites 

 represent approximately, the tenth to the twentieth pairs, therefore 

 all of the somites between the first and tenth pairs have been 

 destroyed by injuring the anterior one-third of the primitive 

 streak. The notochord is also lacking in these embryos. 



It is evident from these results that the primitive streak of eigh- 

 teen hours represents the material from which the trunk and tail 

 regions of the later embryo develop; that the posterior third of 

 the primitive streak represents the region back of the eighteenth 



