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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#Unit 11. Evolution and Behavior

The Forces of Evolution

Mutation, 

Genetic drift

Gene flow or migration

Selection

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#Id: 4316

#Unit 11. Evolution and Behavior

Theories to explain origin of life

Special Creation

Panspermia/ Cosmozoic theory/ Interplanetary theory

Spontaneous Generation

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#Id: 4317

#Unit 11. Evolution and Behavior

The Theory of Spontaneous Generation

The Greek philosopher Aristotle (384–322 BC) was one of the earliest recorded scholars to articulate the theory of spontaneous generation, the notion that life can arise from nonliving matter.

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#Unit 11. Evolution and Behavior

Aristotle proposed that life arose from nonliving material if the material contained pneuma (“vital heat”).

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#Id: 4319

#Unit 11. Evolution and Behavior

As evidence, he noted several instances of the appearance of animals from environments previously devoid of such animals, such as the seemingly sudden appearance of fish in a new puddle of water.

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#Id: 4320

#Unit 11. Evolution and Behavior

When the roof leaked and the grain molded, mice appeared. Jan Baptista van Helmont, a seventeenth century Flemish scientist, proposed that mice could arise from rags and wheat kernels left in an open container for 3 weeks.