BLASTODERM 



BLASTOCOEL 



MARGINAL PERIBLAST 



Fig. 119. The early blastoderm of the hen's egg. Top: A section through the 

 blastoderm, showing many cells which form the roof of a small cavity, the 

 blastocoel. The floor of the blastocoel is non-cellular yolk. The wide circle repre- 

 sents the yolk within the vitelline membrane. During development the blastoderm 

 spreads over the surface of the yolk in the direction of the arrows. Center: A 

 surface view of the blastoderm. Bounding the cellular portion of the blastoderm 

 is a ring of protoplasm. This is the periblast, a ring of non-cellular protoplasm 

 containing nuclei. As this protoplasm is incorporated into cells the area of the 

 blastoderm increases. 



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