EARLY DEVELOPMENT OF THE CHICK. 



79 



tends laterally from the notocord (which here separates the sheet into the 

 two lateral portions) and forward like a horn over the area opaca on each 

 side. Over a portion of the area opaca the network of blood islands and 

 vessels forms a prominent differentiated part of the mesoderm. 



\/ From this time on, the changes in the mesoderm are rapid and extensive. 

 The cells multiply rapidly, resulting in a thickening of the layer generally. 

 Along each side of the notocord the paraxial? portion becomes distinctly 

 thicker and then shades off into the thinner lateral portion. A short dis- 

 tance ahead of the primitive streak the paraxial band becomes marked off 



Neuropore 



Fore-brain vesicle 



Head fold 



Area pellucida 



Area vasculosa 

 Area opaca 



Yolk 



Edge of 

 blastoderm 



~ Proamnion 



Mid- and hind- 

 brain vesicles 



Neural fold 



PFic. 51. Dorsal view of chick embryo with ten pairs of mesodermal somites. Bonnet. 

 :ransversely into blocks by a loosening of the cells between the blocks. 

 These are bilaterally symmetrical and are at once recognized as mesodermal 

 somites (Fig. 51). The pair that appears first is the second pair of somites 

 in the series, the first pair of the series being represented by a close aggre- 

 gation of cells in front of the first cleft. This has been shown experimentally 

 by delicately injuring the first pair with an electric needle and then allowing 

 the blastoderm to continue to develop for several hours; it was found that 

 the other somites of the series appeared behind the injury. A third pair 



