TYPE OF MARKING GENERALIZED RESULTS 



NUMBER NUMBER 



MARKED 



DE V E L. 



35 



m 



rz" 



31 



21 



TOTAL 



I 95 



Fig. 206. Recession of the primitive streak of the chick and growth of the embryo in 

 front of Hensen's node. Marked cell groups represented by heavy dots; dashes opposite 

 these are reference marks placed on the plasma clot to permit orientation. This diagram 

 based upon 6 different types of carbon-marking experiments with the generalized results 

 6-15 hours following explantation. In type I, the stippled area is invaginated as indicated. 

 Observe especially type VI, the history of the three areas marked by the three heavy 

 dots placed on the blastoderm at the head-process stage. It is to be observed that the 

 embryo as a whole arises from the area in front of Hensen's node. (After Spratt, '47.) 



430 



