Appendix A. 
161 
the lab by collecting some cells when they are about to go through 
mitosis so their chromosomes can be stained and be spread out so 
their number and appearance can be examined. A normal set of 
chromosomes produces a characteristic picture (22 recognizable 
pairs and a pair of sex chromosomes) called the normal karyotype. If 
a cell is aneuploid, it will produce an abnormal karyotype picture. If 
the aneuploid cell becomes an egg or sperm and is then involved in a 
conception, the embryo is also aneuploid. Aneuploidies are not 
uncommon events in germ cell development, but aneuploid survival 
is uncommon; nearly all aneuploidies are fatal very early in 
development. 
n. FERTILIZATION AND CLEAVAGE 
Like the word “embryo," the word "conception" refers to a series 
of events or processes, not an instantaneous occurrence. Human 
development begins after the union of egg and sperm cells during a 
process known as fertilization. Fertilization itself comprises a 
sequence of events that begins with the contact of a sperm cell with 
an egg cell and ends with the fusion of their two pronuclei (each 
containing 23 chromosomes) to form a new diploid cell, called a 
zygote. Fertilization normally occurs in the ampulla of the uterine 
tube 12-24 hours after ovulation (Figure 4). 
Before that, however, sperm must travel through the vagina and 
the cervix, through the uterus, and then up the uterine tube. Smooth 
muscle contractions in the uterine tubes as well as ciliary activity 
(waving of hair-like structures) of the tube's lining both are important 
in the transport of sperm up, and of the ovum into and then down, 
the uterine tube. Many more sperm, on the order of tens, or even 
hundreds, of millions, are ejaculated than reach the ovum. Those 
sperm that do come into the vicinity of the ovum must get through 
the material covering the ovum (the corona radiata and the zona 
pellucida) and finally contact and bind to the ovum's membrane, by 
means of specialized structures in the head of the sperm cell. When a 
sperm does get into the ovum, then the ovum membrane changes so 
that other sperm cannot enter. Meanwhile, the sperm cell in the egg 
is also undergoing changes and its specialized structures fall away. 
The haploid nuclei of both the sperm and the egg are now called 
male and female pronuclei. Both swell, as their densely packed DNA 
loosens up prior to replication, and they also migrate toward the 
center of the ovum. Then their nuclear membranes disintegrate and 
the paternally and maternally contributed chromosomes pair up, an 
event called 
PRE-PUBLICATION VERSION 
