EARLY DEVELOPMENT OF THE CHICK. 



77 



exemplifies again the general principle that growth and differentiation in the 

 early stages proceed from before backward. 

 x| Origin of the Mesoderm. The 

 presence of the mesoderm between 

 the other two layers in and lateral 

 to the primitive streak has already 

 been noted (Fig. 47). The cells 

 composing the mesoderm appear to 

 arise in the streak and migrate lat- 

 erally as irregular elements which are 

 so scattered that they do not at first 

 form a complete layer. Whether 

 they originate from ectoderm or 

 entoderm is difficult to determine. 

 The interpretation by those who 

 have studied the problem most care- 

 fully is that the early mesoderm cells 

 originate and differentiate from the 

 thickened ectoderm along the prim- 

 itive groove. The migrating cells 

 multiply rapidly and soon a complete 

 layer is formed which extends across 

 the pellucid area until its margin 

 overlaps the opaque area. The 

 growth of the mesoderm is at first 

 most rapid around the caudal end of 

 the primitive streak, then it extends 

 across the clear area laterally, and 

 finally reaches forward on the two 

 sides as horns which meet in front of 

 the developing embryo but leave an 

 area (the proamnion) in the head 

 region unoccupied by mesoderm until 

 much later. 



When the mesoderm overlaps the 

 opaque area this area thus becomes 

 three-layered, comprising ectoderm, 

 mesoderm and germ wall. The meso- 

 derm, if it does not actually merge 

 with the germ wall, at least establishes intimate contact with it. While the 

 mesodermal cells that arose in the primitive streak continue to proliferate, 



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