The Location of the Chick Embryo. 377 



slight before the appearance of the primitive streak when the area 

 becomes pear-shaped. 



Kopsch^ has found that when two wounds are made in an em- 

 bryo of twenty-four hours' incubation, at a distance of 2 mm., one 

 at the anterior, and the other at the posterior end of the primitive 

 streak, the embryo does not reach its normal size in later develop- 

 ment. The entire body is much shortened, and lies between the 

 two wounds. I have repeated this experiment, and have obtained 

 the same result, the primitive streak, in the eggs upon which I 

 have worked, is much longer (3 to 3.5 mm.) in a twenty-four hour 

 chick, and the anterior end is no longer visible, the head process 

 and the notochord are present. 



Another series of experiments was made by Kopsch when the 

 primitive streak measured about 4 mm. A series of five injuries, 

 at 1.5 mm. spaces, were made along the side of the primitive 

 streak, and parallel with it. The embryos were incubated fifty 

 and one-half hours, and at the end of this time the regions of the 

 five wounds were located. Growth in length was greater in the 

 region back of the anterior end of the primitive streak than it was 

 in front of it. 



I have already described experiments which I have made upon 

 the primitive streak and have tried to show that the anterior end 

 of the primitive streak of sixteen to eighteen hours represents the 

 region of the later embryo which lies back of the heart between the 

 anterior somites. 



Experiment V. These experiments were repeated with some 

 modifications. Instead of injuring the anterior end alone, a 

 second wound was made at the posterior end (Text-fig. 10, x and 

 y). The embryo at the time of the operation was from sixteen to 

 eighteen hours old. After forty hours a normal embryo developed 

 but instead of extending posteriorly to the usual length it was 

 shortened 2 mm. Another egg injured in the same way (Text- 

 fig. 10) developed into an interesting embryo (PI. II, Fig. 10). 

 The posterior wound {y) healed so that no trace of it could be 

 discovered, but the anterior wound (x), through the further 



^Kopsch. Loc. cit. 



