The Location of the Chick Embryo. 379 



metrical. From the twelfth to the eighteenth hour the area pellu- 

 cida increases in length posteriorly. From the eighteenth to the 

 twenty-fourth hour growth continues posteriorly and also proceeds 

 in an anterior direction. From the end of the first day to the end 

 of the second day it advances from the heart in both directions, 

 more rapidly caudad than cephalad. After this time the tail and 

 head are folded off from the surface of the blastoderm. 



III. THE ORIGIN OF THE MATERIAL FROM WHICH THE LATER 



EMBRYO ARISES. 



I have already spoken of Kopsch's conclusions as to the material 

 from which the embryo arises, so that I shall merely mention my 

 own results. If, as Kopsch says, the primitive streak represents 

 the entire embryo with the exception of the pre-chordal head 

 region, then the destruction of definite areas of the primitive 

 streak should result in a failure to develop the parts which arise 

 from the injured area. 



Experiment I. The first experiment consisted in destroying all 

 of the primitive streak except its anterior end (Text-fig. 12). 

 This operation is very likely to kill the entire embryo as injury 

 to so large an area usually results in a spreading apart of the mar- 

 gins of the wound. The further development of an embryo 

 injured in this way may be seen in PI. II, Fig. 12. The embryo 

 is abnormal, but shows structures which indicate that when de- 

 prived of all of its material except the anterior end the primitive 

 streak gives rise to the first few pairs of somites; and that the brain 

 and notochord develop. The somites are much thinner than in 

 the normal embryo. 



Experiment II. In another series of experiments the posterior 

 third of the primitive streak was destroyed (Text-fig. 13). 

 The destruction of this region resulted in an embryo (PI. II, Fig. 

 13), in which the entire caudal region was abnormal. The heart 

 and brain, which are not represented in the figure, were normal, 

 and fifteen to eighteen pairs of somites were formed in the anterior 

 trunk region. This result agrees with Kopsch's view that the 

 posterior third of the primitive streak represents the caudal region 

 of the embryo from the twentieth somite to the posterior end. 



