164 
Monitoring Stem Cell Research 
the most basic manifestations of emerging specialization. But the 
egg is roughly spherical, and it is not readily apparent how polarity 
is established. Although it had been shown long ago that the point of 
sperm entry determines the plane of first cleavage (and thus 
subsequent ones) in amphibian eggs, mammals were believed until 
recently to remain spherically symmetrical until later in development. 
Recent data on mammalian zygotes, however, suggests that the 
point of sperm entry may similarly determine the cleavage plane.^ 
Even the first two cells resulting from the first cleavage may have 
different propensities, which persist through the next divisions as 
the progeny of one cell tend to become the body of the offspring and 
progeny of the other cell become the embryo’s contribution to the 
placenta and other supporting structures. The word “fate,” however, 
might be too strong, because the cells of such very early embryos are 
resilient to perturbations — if one cell is removed, the remaining ones 
can compensate. 
in. IMPLANTATION 
After fertilization, the zygote proceeds immediately to the first 
cleavage and subsequent cell divisions follow rapidly. The zygote is 
a very large cell, but the first waves of rapid cell division occur 
without increase in cell volume. The result is a closely bound mass of 
cells each of more typical cell size. At this stage the cells are called 
blastomeres, (“parts of the blast,” “blast” coming firom the Greek for 
"bud” or “germ”) and the organism as a whole is called a morula 
(from the Latin for mulberry, descriptive of its appearance) firom the 
time it has 16 blastomeres to the next stage. The morula is still 
encased in the zona pellucida. As it is undergoing this very rapid cell 
division, the organism is also migrating down the uterine tube 
toward the uterus. After it arrives in the uterus, at about day five 
after the initation of fertilization, the zona pellucida breaks up; the 
process is called “hatching” and is a necessary prelude to 
implantation. 
Many zygotes do not survive this long. Estimates vary widely of 
the rate of natural embryo loss prior to implantation or after 
implantation but still early in gestation. One study of healthy women 
trying to conceive found 22 percent of pregnancies (identified by 
sensitive hormone measures) were lost prior to becoming detectable 
clinically. Even after implantation, there is a substantial rate of loss, 
still not known precisely but estimated at 25 to 40 percent.^ 
When the morula enters the uterus, fluid starts to accumulate 
between its blastomeres. The fluid-filled spaces run together, 
forming a relatively large fluid-filled cavity. At the point when the 
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