Appendix A. 
Notes on Early Human Development 
The term “embryo" refers to an organism in the early stages of its 
development. In humans, the term is traditionally reserved for the 
first two months of development. After that point, the term "embryo" 
is replaced by the term “fetus" which then applies until birth. Some 
authors further reserve the term “embryo" for the organism only after 
it has implanted and established its placental connection to the 
pregnant woman. Similarly many also reserve the term “pregnancy" 
for the state of the woman only after implantation. At the beginning 
of the individual’s development, the entity is a single cell. After two 
months, it has limbs, distinct fingers and toes, internal development, 
and countless cells. So the term “embryo" applies to an individual 
throughout a vast range of developmental change. This document is 
a description of early human development, with emphasis on those 
events or structures that have figured most prominently in recent 
discussions of research using human embryos or their parts, 
especially for stem cell research.^ 
Development has fascinated centuries of observers, as they 
pursued deeper understanding of the stability of species 
characteristics at least from one generation to the next, as well as 
the uniqueness of each offspring. Uniqueness is especially marked in 
sexually reproducing organisms, that is, organisms where the 
genetic make-up of the offspring comes from a combination of 
maternal and paternal DNA, because a new genome is formed in 
each instance of conception. The stability reflects inheritance 
connecting one generation v\hth the past and future members of its 
line. 
Organisms and the processes of their development have evolved. 
As a result, the development of any organism has a species-specific 
pattern, but also shares many of the same developmental processes 
v\nth other species related from its evolutionary origins. Many of the 
processes discussed here are common not just to all humans, or to all 
mammals, but to all vertebrates. In some cases they are shared even 
with invertebrates as well. 
The process whereby a new individual of the species comes into 
being has been at the center of too many deep inquiries to list here, 
let alone discuss in the depth they deserve. But even in this short 
document it is important to note one question that is related to the 
connection of one generation to the next and previous generations. 
That is, how are we to understand the apparent directedness of 
development, following a complex network of pathways from a single 
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