The Location of the Chick Embryo. 381 



pair of somites, the middle third represents roughly, from the 

 twelfth pair to the eighteenth, while the anterior third supplies 

 material for those structures which lie between the heart and the 

 twelfth pair of somites, but does not include the chordal region of 

 the brain. 



SUMMARY AND CONCLUSIONS. 



I. The central point of the unincubated blastoderm represents 

 the anterior end of the primitive streak, and later, the region just 

 back of the heart; therefore, the greater part of the embryo 

 develops in the posterior half of the blastoderm. 



2. The region midway between the center of the unincubated 

 blastoderm and its anterior border represents the head region of 

 the later embryo. 



3. The position of the embryo on the area pellucida is fixed. 

 The long axis of the future embryo divides the unincubated 

 blastoderm into right and left halves and a line drawn through the 

 blastoderm in the long axis of the shell divides it into anterior and 

 posterior halves. 



4. Destruction of the material of the unincubated blastoderm 

 between the center and its posterior margin does not result in the 

 formation of the primitive streak on any other radius. 



5. The growth of the blastoderm is uniform up to the eighth 

 to tenth hour, and this uniformity is preserved in the later growth 

 of the area opaca, but from the tenth hour the area pellucida 

 begins to grow more rapidly in a posterior direction, then later it 

 advances anteriorly until it assumes an oval form. Up to the third 

 day the region immediately back of the heart (the anterior end 

 of the early primitive streak) is the center of growth in all four 

 directions, anteriorly, to the left, and to the right, and to a much 

 greater extent posteriorly. 



6. Injury to the center and posterior margin of the unincubated 

 blastoderm results in a shortened embryo. 



7. Injury at the posterior margin alone will shorten the embryo 

 by preventing growth in a posterior direction. 



