DEVELOPMENT OF CALLUS 



201 



A thickening of the 

 upper layer of the blasto- 

 derm appears in the 

 centre of the area pel- 

 lucida, in the form of a 

 straight band stretching 

 at right angles to the 

 line passing through the 

 pointed to the blunt end 

 of the " Qgg^ This is 

 the primitive streak, the 

 first differentiation of the 

 embryo, which will be 

 formed along its axis. 

 When an observer holds 

 an egg in front of him 

 with the blunt end to the 

 left, the axis of the embryo 

 will therefore run straight 

 in front of him, and the 

 embryo is so orientated 

 that its head is away from 

 the observer, and its tail 

 towards him. 



Running along the 

 middle line of the primi- 

 tive streak is a shallow 

 groove on the surface, 

 the primitive groove, 

 which runs into a small 

 depression at the front 

 end of the primitive streak 

 known as the primitive 

 pit. Immediately in front 

 of the primitive pit is a 

 slight rise, forming the 

 so-called primitive knot. 

 Now the primitive knot 



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A -° 



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