In any particular cell, most genes are expressed at a low level

Only a small number of genes, whose products are specialized for the cell type, are highly expressed.

mRNAs expressed at low levels overlap extensively when different cell types are compared.

The abundantly expressed mRNAs are usually specific for the cell type.

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#Id: 1590

#Unit 5. Developmental Biology

Primitive groove is homologous to the amphibian blastopore

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#Id: 1591

#Unit 5. Developmental Biology

Hensen’s node (Primitive Knot)

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#Id: 1592

#Unit 5. Developmental Biology

A regional thickening of cells at the anterior end of the primitive streak

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#Id: 1593

#Unit 5. Developmental Biology

Hensen’s node is the functional equivalent of the organizer (dorsal lip of the amphibian blastopore)

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#Id: 1594

#Unit 5. Developmental Biology

Primitive Pit 

A funnel-shaped depression at the center of Hensen’s node through which cells migrate to form the notochord and prechordal plate.

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#Id: 1595

#Unit 5. Developmental Biology

The primitive streak defines the major body axes of the avian embryo.