Lecture 7b: Comparative Embryology

Preformationism

Old hypothesis for devlopment

• adult organism is preformed in sperm

• called homunculus in humans

• development is then just growing

Epigenesis

• adult organism is not preformed

• devlopment is a process of transformation

• first proposed by aristotle after observing bird eggs

William Harvey Deer Experiments

• mated deer then looked for embryos in uterus at different times

• lent support to epigenesis

von Leuwenhoek

• microscopist

• discovered sperm in human semen

• thought they were parasites

• believed animacules fed on eggs to develop

Oskar Hetwig and Herman Fol

• independently observed fertilization in sea urchans

• recognized that fertilzation was the fusion of sperm and egg

• noticed only one sperm entered each egg

• saw the two pronuclei unite

Prevost and Dumas

• 1824 describe cleavage in eggs

Carl Bergmann

• came to the conclusion that egg furrows eventually became cells of embryo

• correctly identified the nuclei during cleavage

• laid groundwork for understanding developments organization of functionally specialized cells

Karl Ernst von Baer

• described blastula in detail

• discovered the notochord

• helped formulate the germ-layer theory of development

• discovered mammalian ovum

Primary embryonic axis

Phases of Devlopment

• gametogenesis

• fertilization

• cleavage

• gastrulation

• organogenesis

Cleavage

initial cell multiplication

• gives rise to blastula

Cleavage types

Gastrulation

• origin of germ layers

• gives rise to gastrula with a blastopore and archenteron (gut cavity)

• proceeds by invagination, epiboly, ingression

• blastopore can become mouth, anus or neither