He argues for reductionist development convincingly, specifically by examining development based on local interactions. My only question may be just a small and unnecessary one: - Are these local, repetitive interactions all that simple? Whether physical or chemical, or whether momentary or some constant gradient, the interactions and influences on each cell/portions of each cell are complicated and oftentimes competing. Because the entire mechanism is likely sensitive, I suspect the local interactions are more nuanced than suggested in the article. Perhaps he is only suggesting that relative to other possible mechanisms for organizing such a phenomenon, the local interactions are simple? I could see that being true, but in an absolute sense, the interactions are complicated enough for us to be unable to fully simulate them (though not so complicated we can't understand their basic workings).

This description of the development of eye cells reminds me of some fascinating reading I did awhile back. Like the pulses of spineless that color receptors in the fly’s eye cells, the bands of light (yellow-red) and dark (black-brown) pigment in the hairs of agouti (banded hairs of wild-type) animals are also caused by a switching off & on – in this case of the Agouti protein which suppresses the reaction (a binding of MSH to a melanocortin receptor on the malanocyte) that causes brown-black pigment on hairs. The most interesting part (to me) about this set of pages is the description of the relationship between pigmentation and temperament. “The neurotransmitter dopamine and the hormones noradrenaline and adrenaline, which are involved in the stress response, have the same biochemical precursor as the melanin pigments.” Moreover, there are also melanocortin receptors on neural cells which function as neuromodulators, and the Agouti protein does the same thing there – stops MSH from binding to them. It’s believed that this variance in the level of MSH is what causes the difference in behavior between agouti and non-agouti animals. Sure enough, it’s been shown time & again that animals with agouti coloring are much less docile and quicker to frighten than solid or spotted animals. Wild animals bred for domestication lose their agouti coloring in the process. The case examples are great reading. There are a lot of other relationships described here – albinism and blindness, agouti and food intake, etc. What a terrific reminder that we are emergent systems – that the interactions between the basic parts of the system at step one create the diversity we see at the next step in development (and on and on), and that what appear to be discrete parts of the system are in fact still related in complex (and interesting) ways.