156 
Monitoring Stem Cell Research 
cell to a multi-system, free-living, and even conscious being? This 
process occurs in a reliable pattern time after time, but also is 
sufficiently resilient to perturbations that developing entities can 
recover from significant disturbances. For example, at early stages of 
development an embryo may divide (or be cut) completely in half, 
and then each half recovers to form an entire offspring, resulting in 
identical twins. 
Different notions of purposive directedness, functional 
explanation, and even vital forces have been invoked to explain 
development. One of the insights, from the relation of development to 
evolution, is that the development of an individual reflects the fact 
that it is descended from individuals that reproduced successfully 
and, like its forebears whose DNA it inherited, its development 
reflects their past survival with their particular characteristics. This 
legacy of ancestral success at survival is manifested in the new 
organism’s apparent directedness toward development along hnes 
that enhance its own survival. Even very early embryos follow 
patterns of differentiation in the progeny of different cells. These 
patterns, in embryology, are called the fate of the progeny of a cell. 
The fate of the progeny of the newest single cell embryo is maximally 
broad — if it survives it will give rise to every type of cell of the 
species. But as the embryo becomes multicellular, its cells specialize 
and, in the absence of artificial perturbation, their progeny have 
increasingly specialized fates as well. 
The evolved events and processes of development include some 
that reflect distant relations, such as the yolk sac that is conserved in 
placental mammals including human beings. Other events or 
processes exhibit the evolution of more specific characteristics. In 
animals such as human beings, the speciahzed and complex 
membraneous structures that form the connection between the 
individual body of the pregnant woman and the developing 
individual body of the offspring begin to arise in the first week. 
Human embryos implant in the uterine wall starting at about the 
sixth day after conception, so of course they must arrive in the uterus 
with membranes capable of participating in that bond. They do not 
have a fully formed placenta at such an early stage, nor is the uterine 
wall unilaterally ready, but rather the contact of embryo and 
endometrium initiates complementary development finally resulting 
in the fully developed placenta. One way to look at it is that the early 
embryo's very structure points to the future, showing its overall 
developmental fate to be connected to the maternal body. Another 
perspective is that this process reflects the past survival of many 
generations. In both senses, no moment of development can be 
understood in isolation from the context of the organism’s reflection 
PRE-PUBLICATION VERSION 
