DEVELOPMENT OF GALLUS 



203 



cells which grow out laterally between the upper and lower 

 layers. The primitive knot produces tissue which gets pushed 

 forwards underneath the upper layer in the middle line, and 



pa 





Mist M'.'O'."- ' 'rfi-jsa 



"tiBm 



'.#**itmgi 



FlG. 91. — Gallus : view of the blastoderm of a hen's egg after 24 hours' 

 incubation, as seen by transmitted light. (After Jenkinson.) 



fg, foregut ; m, mesoderm ; ms, mesodermal somites ; tip, neuropore ; 

 vv, vitelline vein. Other letters as Fig. 90. 



gives rise to the notochord. During development the primitive 

 streak moves back along the blastoderm, leaving in front of it 

 a trail of cells destined to become notochord, and on each side 



