lacks a tail. Its facial features have also started to form. The older embryo is much bigger,
as well.
The pictures in theTable(22.1) show how a developing baby looks, beginning at 4 weeks
after fertilization and ending at 38 weeks after fertilization. The pictures show the baby at
the same size at each stage. However, the actual size increases greatly during development.
The length of the baby at each stage is given in the figure. Read the information in the
boxes to learn what organs and other features have developed by each stage.
Table 22.1: Human EmbryoImage Developmental characteristics- Facial features are just starting to
form. - Tail is present.
- Legs have formed, and arm buds have
appeared. - Heart is partly formed and begins to
beat. - Spinal cord and brain have started to
develop. - Most other organs have started to
form, including the liver, pancreas,
gall bladder, spleen, and lungs. - Facial features are starting to look hu-
man, and external ears and eyes are
beginning to form. - Tail has disappeared.
- Arms have developed; fingers and toes
are starting to form. - Heart is well developed.
- Digestive system is developing rapidly
but does not yet function. - Cartilage and bones have started to
form, and muscles are developing.