 Alcmaeon is the first to raise a series of questions in human and animal physiology that later become stock problems, which every thinker tries to address. He thus sets the initial agenda for Greek physiology (Longrigg 1993, 54–7; Lloyd 1966, 322 ff.). He said that sleep is produced by the withdrawal of the blood away from the surface of the body to the larger (“blood-flowing”) vessels and that we awake when the blood diffuses throughout the body again (DK, A18). Death occurs when the blood withdraws entirely. Hippocratic writers (Epid. VI 5.15) and Aristotle (H.A. 521a15) both seem to have adopted Alcmaeon’s account of sleep. It is very unlikely, however, that Alcmaeon distinguished between veins (the “blood-flowing vessels”) and arteries, as some have claimed. It is more likely that he simply distinguished between larger more interior blood vessels as opposed to smaller ones close to the surface (Lloyd 1991, 177). He probably argued that human seed was drawn from the brain (DK, A13). It has recently been suggested (König 2019), however, that he instead thought 1) that sperm came from the whole body, 2) that the brain was its transit port and 3) that it was ultimately a product of digestion. But no ancient source directly assigns any of these doctrines to Alcmaeon and the arguments in favor of them are very indirect. Contrary to a popular Greek view, which regarded the father alone as providing seed, a view that would be followed by Aristotle (Lloyd 1983, 86 ff.), Alcmaeon may have argued that both parents contribute seed (DK, A13) and that the child takes the sex of the parent who contributes the most seed (DK, A14). It has recently been suggested, however, that our sole source for these views (Censorinus) is mistaken and that, while Alcmaeon thought that both the male and female contributed to the child, only the male contributed seed (Leitao 2012: 278–79), the female contributing menses. According to one report, Alcmaeon thought that the head was the first part of the embryo to develop, although another report has him confessing that he did not have definite knowledge in this area, because no one is able to perceive what is formed first in the infant (DK, A13). These reports are not inconsistent and conform to the epistemology with which Alcmaeon began his book. It is plausible to suppose that he regarded the development of the embryo as one of the imperceptibles about which we can have no certain knowledge. On the other hand, he may have regarded it as a reasonable inference that the part of the body which controls it in life, i.e., the brain, developed first in the womb. Dissection is of obvious relevance to the debate about the development of the embryo, and Alcmaeon’s failure to appeal to dissection of animals in this case is further evidence that he did not employ it as a regular method (Lloyd 1979, 163). Alcmaeon studied not just humans but also animals and plants. He gave an explanation of the sterility of mules (DK, B3) and, if we can believe Aristotle, thought that goats breathed through their ears (DK, A7). More significantly, he used analogies with animals and plants in developing his accounts of human physiology. Thus, the pubic hair that develops when human males are about to produce seed for the first time at age fourteen is analogous to the flowering of plants before they produce seed (DK, A15); milk in mammals is analogous to egg white in birds (DK, A16). The infant in the womb absorbs nutrients through its entire body, like a sponge, although another report suggests that the embryo already feeds through its mouth (DK, A17). However, the text of the latter report should perhaps be amended from stomati [mouth] to sômati [body], thus removing the contradiction (Olivieri 1919, 34). Analogies such as these will become a staple item in later Greek biological treatises, but Alcmaeon is one of the earliest figures in this tradition.