A Lesson from Anatomy: hints of greater things

I am looking at a book called Developmental Anatomy by L. B. Arey, doing a bit of research. This chapter is on the Vascular System (the blood, blood vessels, and heart).

You may already know that our red blood cells do not have a nucleus; they do NOT divide. They spend their lives carrying oxygen to the rest of the body, and taking away carbon dioxide. The curious thing is that for some time in embryonic life, there are blood cells which reproduce:

At seven weeks almost every red blood cell in the circulating blood is nucleated; at 11 weeks only a few with nuclei remain.
[Arey, Developmental Anatomy 346]

Note that the times are measured from conception.

Here's another important datum:

As a mammalian embryo advances through the stages characterized by cleavage, morula, blastocyst and germ layers, it satisfies all its metabolic needs by simple, diffusive interchanges with the fluid medium inwhich it is immersed. But as the embryo continues to gain size and begins to take form, a functioning circulatory system becomes necessary in order to make use of the required food and oxygen obtainable from the mother's blood. Hence it is that the heart and blood vessels are the first organ system to reach a functional state.
[Arey, Developmental Anatomy 375]

For reference, the beat begins at about 3.5 weeks; the general form of the heart is complete by about 6 weeks.