Development of the First Day 125 



The meaning of the primitive streak and 

 groove has been much discussed. It is now 

 generally considered to correspond, in part 

 at least, to the elongated lips of the blasto- 

 pore in the frog, that have come together, 

 on the closure of the blastopore, as has been 

 described in a previous part of this book. 

 The primitive groove would correspond to 

 the line of fusion of the two lips of the 

 blastopore. 



The mesoblast in the chick seems to be 

 derived from three distinct sources, though 

 it is not easy to make out these points. 



One of these sources has been mentioned 

 in the primitive streak. A second source is 

 the scattered group of cells that was left 

 between the ectoblast and entoblast on the 

 formation of the latter as a distinct layer of 

 cells. In the middle and lateral parts of the 

 area pellucida, at about the time of the form- 

 ation of the primitive streak, cells are budded 

 off from the upper side of the entoblast and 

 become mesoblast. 



The mesoblast cells, derived from these 

 three sources, very soon unite to form a con- 

 tinuous layer, so that it is impossible to tell 

 from which source any particular cell was 



