The Location of the Chick Embryo. 375 



While the margin of the area opaca is symmetrical, that of the 

 area pellucida is not. During the first few hours of incubation 

 the two areas increase uniformly, but towards the fifteenth hour 

 the area pellucida begins to extend posteriorly, the anterior region 

 remaining spherical in outline. 



Experiment /. The uniformity of growth in the anterior half 

 of the blastoderm can be seen in the following experiment. The 

 unincubated blastoderm was injured at three points (Text-fig. 7, 

 X, p and 0) ; the needle was inserted at the center (x) at the middle 

 point of the anterior margin (/>) and at the right margin of the 

 area pellucida (0). The injuries in the margin were at equal 

 distances from the center. After eighteen hours' incubation the 

 distance between x and p was the same as the distance between x 

 and (PI. I, Fig. 5) showing that the lateral and anterior growth 

 were the same. The primitive streak was formed, but its posterior 

 end (jy) was much further from x than x was from p, while the 

 distances before incubation were equal. 



The results of earlier experiments^ led me to believe that the 

 region immediately in front of the primitive streak represents an 

 area of rapid growth, because an injury made in this region did 

 not affect one structure alone, but disturbed the organ covering a 

 large area. This is also true when the center of the unincubated 

 blastoderm is killed (PI. I, Fig. 2). 



Experiment II. In order to determine the extent of growth in 

 an interior direction from the center of the blastoderm I injured 

 a point in the center (Text-fig. 8, x) and one on the same level at 

 the side (o). The eggs were incubated thirty-six to forty hours. 

 PI. I, Fig. 6 is a surface view of an embryo at the end of thirty 

 hours. The injury in the center of the blastoderm produced 

 great disturbance in the development of the embryo anterior to 

 the heart. No forward growth took place in the median line. 

 The wound (0) at the side, which did not move forward, is at a 

 level with the anterior somites. The normal growth of the margin 

 of the area pellucida on the side of the injury did not take place. 

 The margin is irregular and a peculiar rod of cells extends from 

 the marginal wound to the median line of the embryo. 



^Peebles. Loc. cit. 



