43. Animal Development:
From Genes to Organisms 920
43.1
How Does Fertilization Activate Development?
921
The sperm and the egg make different
contributions to the zygote 921
Rearrangements of egg cytoplasm set
the stage for determination 922
Cleavage repackages the cytoplasm
922
Cleavage in mammals is unique 923
Specific blastomeres generate specific
tissues and organs 925 43.2
How Does Gastrulation Generate Multiple
Tissue Layers? 926
Invagination of the vegetal pole characterizes
gastrulation in the sea urchin 927
Gastrulation in the frog begins at
the gray crescent 928
The dorsal lip of the blastopore organizes
embryo formation 928
The molecular mechanisms of the organizer
involve multiple transcription factors
929
The organizer changes its activity
as it migrates from the dorsal lip
931
Reptilian and avian gastrulation is
an adaptation to yolky eggs 931
Placental mammals have no yolk but
retain the avian–reptilian gastrulation
pattern 932
43.3
How Do Organs and Organ Systems Develop?
932
The stage is set by the dorsal lip
of the blastopore 932
Body segmentation develops during
neurulation 933
Hox genes control development along
the anterior–posterior axis
933
43.4
What Is the Origin of the Placenta?
934
Extraembryonic membranes form with
contributions from all germ layers
934
Extraembryonic membranes in mammals
form the placenta 935
The extraembryonic membranes provide
means of detecting genetic diseases
936
43.5
What Are the Stages of Human Development?
936
The embryo becomes a fetus in the
first trimester 936
The fetus grows and matures during
the second and third trimesters 937
Developmental changes continue throughout
life 938
