Entwickslungmechanik
Developmental Mechanisms
Wilhelm Roux:
“We must not hide from ourselves the fact
that the causal investigation of organisms is
one of the most difficult, if not the most
difficult, problem which the human intellect
has attempted to solve… since every new
cause ascertained only gives rise to fresh
questions regarding the cause of this cause.”
Experimental Embryology
 Mechanics
of Cell Specification
 Mechanics
of Morphogenesis
Experimental Embryology
Experimental embryology had its
beginnings in the testing of hypotheses of
cell specification
Experimental Embryology
August Weismann’s Germ Plasm Theory
 1883
 First testable hypothesis of cell
specification
 Determinants within zygote were
partitioned into cells as development
proceeded
Experimental Embryology
Weismann’s Hypothesis
Determined
Cells
Differentiated
Cells
Experimental Embryology

Weismann hypothesis




Egg and sperm provide equal chromosomal
contributions to embryo
Chromosomes carried the inherited determinants
The chromosomes/determinants were somehow
differentially distributed to embryonic cells
Only the germ cells received all determinants
(germ plasm)
Experimental Embryology
Testing Weismann’s Hypothesis
 Each half of a developing frog embryo is
derived from one cell of the two cell
staged embryo
 Each blastomere are the two cell stage
must therefore contain left and right
determinants
Experimental Embryology
4 Categories of Experiments
 Defect
experiment
 Isolation
experiment
 Recombination
experiment
 Transplantation
experiment
Experimental Embryology
Wilhelm Roux (1888) 1st to test Weismann’s hypothesis.
Roux’s defect experiment supported mosaic development
and partitioning of determinants.
Experimental Embryology
Hans Driesch’s (1892)
Isolation Experiment:
Dissociation of 4 and 8
cell sea urchin embryos
demonstrated regulative
development.
Dissociation was
accomplished by
incubation in Ca free
water implies that Ca is
important for cell
adhesion
Experimental Embryology

Roux’s experiment suffered from an
experimental design flaw

By killing one of the two blastomeres, but not
removing it, the remaining dead cell
prevented rearrangement of the living cells to
produce a normal embryo

Driesch’s experiment did not suffer from this
flaw since he had separated all the cells of
the embryo

McClendon (1910) repeated Driesch’s
experiment in frogs with the same result
Experimental Embryology
Major Impact for Embryology
In regulative development, the prospective
potency is greater than the prospective fate