212 FLORENCE PEEBLES. 



lary groove lying anterior to the hair. In those of forty hours' 

 incubation the point of insertion lay at the level of the most an- 

 terior somites. The results of these experiments, as a whole, 

 indicate that when the blastoderm of an unincubated egg is 

 divided into two halves by a plane passed through the center, 

 perpendicular to the long axis of the later embryo, the half an- 

 terior to this plane gives rise to the heart, brain, sense-organs, 

 medulla and fore-gut, and all the rest of the embryo posterior 

 to these organs arises through the activity of the primitive streak 



region. 



The conclusions of Assheton are fully substantiated by the 

 results of the experiments which I made in 1898, although at 

 that time I used embryos in which the primitive streak was al- 

 ready present, or completely formed. I found that an injury 

 made at the anterior end of the primitive streak of the eighteenth 

 hour of incubation, appeared in forty-eight hours in the region 

 just back of the heart, between the first pairs of somites. 



Kopsch has recently described (1902) a series of experiments 

 which he made in order to determine whether or not the actual 

 cell material of the primitive streak becomes changed into the 

 embryo. This, he concludes, is true, and he is able to show, 

 through these experiments, that the primitive streak represents 

 definite parts of the embryo. The most anterior end becomes 

 the chordal region of the head, from the middle portion the 

 somites and trunk develop, and from the posterior third the parts 

 of the embryo caudal to the twentieth pair of somites are formed. 

 He believes that the young primitive streak represents these re- 

 gions in greater concentration. Thus, the entire embryo with 

 the exception of the pre-chordal head area arises directly from 

 the cell material of the young primitive streak. 



Kopsch recognizes, as all must who attempt experiments on 

 the early embryo of the chick, the great difficulties arising from 

 the variation in degree of development at a given hour, and also 

 the great danger, in the living egg of failure in locating the 

 exact region that is to be injured. These difficulties may account 

 in part for the difference in the results obtained by Kopsch, and 

 by Assheton and myself. 



In the spring of 1902 I made a series of experiments in 



