Events associated with the first week of development • Figure 16.12
1. The_____ is the embryonic stage that implants into the uterus.
2. At implantation, the inner cell mass of the blastocyst is oriented
The first week of development is characterized by cleavage of the zygote into a
morula and then into a more complex blastocyst by successive cell divisions. Dur-
ing this change, the embryo travels from the uterine tube into the uterus.
Inner cell mass
Morula (3-4 days postfertilization)
• Successive cell divisions of the
blastomeres produce a solid
sphere of cells called a
• The morula is surrounded by
the zona pellucida and is the
same size as the original zygote
• When the morula enters the uterine cavity,
glycogen-rich fluid secretions from the endometrium
cause the cells to shift around, forming a fluid-filled
. This hollow sphere of
cells is called a
inner cell mass
becomes the growing embryo.
• The outer cell layer, called the
become the fetal portion of the placenta, where
nutrients and wastes will be exchanged with the
• The zygote divides into two smaller
cells called blastomeres.
• Each blastomere divides again to
produce a four-celled
PATH OF SPERM
of secondary oocyte
Many sperm bind to the secondary oocyte.
The enzymes of the sperm’s acrosome digest
layer of granulosa cells that is attached to the
zona pellucida around a secondary oocyte).
Once one sperm penetrates and touches the
egg’s membrane, the zona pellucida changes
chemically and presents a barrier to other
When the sperm enters the secondary oocyte,
meiosis II is triggered.
The nuclei of the sperm and oocyte fuse to
produce a single
V id e o
Frontal section through uterus, uterine tube, and ovary
Endometrium of uterus
• After floating free for 2 days, the blastocyst
attaches to the uterine wall (implantation).
• The blastocyst’s inner cell mass orients toward
Pregnancy Lasts from Fertilization to Delivery