The Location of the Chick Embryo. 371 



Experiment I. A small window was made in the shell of an egg 

 a few hours after it was laid. The blastoderm measured 2.8 mm. 

 in diameter and the area opaca and area pellucida were faintly 

 defined. A hot needle (No. 12) was inserted in the center of the 

 blastoderm (Text-fig. i, x) and quickly withdrawn. The shell 

 was sealed and the egg put in the incubator, the temperature of 

 which varied from 37°-39° Centigrade. At the end of twenty 

 hours the egg was opened, and the blastoderm killed, removed 

 and stained. The primitive streak was clearly defined (PI. I, 

 Fig. i) extending from the posterior margin of the area pellucida 

 to the point of injury (x). The cells around the wound seemed 

 greatly increased in number and showed evidence of forward 

 growth which must have been stopped by the injury. 



Other eggs, injured in the same way (Text-fig. i) were left in 



-f---x 



2 



the incubator for a longer period, from thirty to forty-eight hours. 

 PI. I, Fig. 2 is a surface view of one of these embryos after forty- 

 eight hours' incubation. The embryo is well developed, fourteen 

 pairs of somites are present, and the heart is forming. The injured 

 area lies dorsal to the heart on a level with the anterior somites. 

 The brain region has failed to develop. 



From Assheton's results, and from these just described, we 

 must conclude that the primitive streak and the greater part of the 

 later embryo form from that region of the unincubated blasto- 

 derm which lies behind the center, between it and the posterior 

 margin of the area pellucida. The question arises whether or 

 not the posterior margin of the area pellucida is a fixed region in 

 all eggs, and what the relation of the long axis of the embryo is to 

 the long axis of the shell. In Text-fig. 2, an egg is represented 



