SMB <SS^ VS5/ 
Figure 7B: Photo micrographs of implantation beginning and completed. 
[www.visembryo.com by Houseworks, Inc.| 
V. TWINNING 
The usual case for human beings is for one ovum to be released, 
and if all goes well, fertilized and developed to term. Less commonly, 
more than one ovum may be released and fertilized so that more than 
one embryo develops. These embryos vrould be genetically distinct, 
sharing the uterus during the same gestation period. They will have 
a family resemblance but no more genetic commonality than any 
other set of siblings, and they may be of the same or different sexes. 
These are called dizygotic twins (because they came horn two 
zygotes). More rarely, a single zygote may, during its early 
cleavages, separate completely into two groups of cells. As 
discussed above, the two cells resulting from the first cleavage may 
already have different probable fates, the progeny of one 
contributing to the body and the other to the supporting structures. 
Both, however, at this stage are still totipotent and can, if disrupted, 
go on to generate a full individual organism. If this separation occurs, 
then monozygotic twins may be bom (Figure 8). Monozygotic twins, 
two offspring coming fi:om one zygote, have the same genome and 
are always of the same sex. When the twinning occurs in the first 
cleavages and there are not yet any extraembryonic membranes 
(Figure 8A), the two develop separately as do dizygotic twins, with 
separate amnions, chorions and eventually placentae. If an embryo 
should divide into two later in its development, between about days 
four and eight, the twins will share the same chorion and therefore 
eventually the same placenta, but a separate amnion will form 
around each (Figure 8B). Should an embryo divide later than this, 
between about the ninth and thirteenth days, the resulting twins will 
share the same amnion, chorion, and placenta. It is very rare for 
PRE-PUBLICATION VERSION 
