The following musings concern certain figures that warrant further scrutiny. Through these distillations, I attempt to draw some general conclusions about how the fly's control system operates and why it evolved this way. Some of the annotations also offer historical perspective.

Figure 2.1 One corollary issue (symbolized by the hourglass) is: how do cells measure time over periods longer than a mitotic cycle? Cells presumably need to do so in order to know when to stop dividing and start differentiating. Possible timekeeping devices include the “POU Hourglass,” which limits the number of mitoses in certain neuroblasts in the fly CNS. This clock gauges the declining amount of the POU-domain proteins Pdm-1 and Pdm-2. Mammalian oligodendrocytes use an “HLH Hourglass” that triggers differentiation when the amount of specific HLH-domain proteins drops below a critical threshold. An oscillator based on this sort of mechanism may be involved in vertebrate somitogenesis. Other protein clocks appear to count mitoses and meioses leading to sperm and egg differentiation. Another strategy involves using a cascade of transcription factors to trigger different events in different phases of the cascade. For RNA clocks, and for general discourses on (noncircadian) timekeeping.

A deeper issue concerns how structures are represented abstractly in the genome. Given that we know most of the genes involved in bristle development (cf. App. 3), we can begin to ask how “bristle” is “written” in “gene language.” The answer is not obvious.