in viviparous animals, as said before, the embryo gets its growth through the umbilical cord. for since the nutritive power of the soul, as well as the others, is present in animals, it straightway sends off this cord like a root to the uterus. the cord consists of blood-vessels in a sheath, more numerous in the larger animals as cattle and the like, one in the smallest, two in those of intermediate size. through this cord the embryo receives its nourishment in the form of blood, for the uterus is the termination of many blood-vessels. all animals with no front teeth in the upper jaw, and all those which have them in both jaws and whose uterus has not one great blood-vessel running through it but many close together insteadall these have in the uterus the so-called cotyledons (with which the umbilical cord connects and is closely united; for the vessels which pass through the cord run backwards and forwards between embryo and uterus and split up into smaller vessels all over the uterus; where they terminate, there are found the cotyledons). their convexity is turned towards the uterus, the concavity towards the embryo. between uterus and embryo are the chorion and the membranes. as the embryo grows and approaches perfection the cotyledons become smaller and finally disappear when it is perfected. for nature sends the sanguineous nutriment for the embryo into this part of the uterus as she sends milk into the breasts, and because the cotyledons are gradually aggregated from many into a few the body of the cotyledon becomes like an eruption or inflammation. so long as the embryo is comparatively small, being unable to receive much nutriment, they are plain and large, but when it has increased in size they fall in together.

but most of the animals which have front teeth in both jaws and no horns have no cotyledons in the uterus, but the umbilical cord runs to meet one blood-vessel, which is large and extends throughout the uterus. of such animals some produce one young at a time, some more than one, but the same description applies to both these classes. (this should be studied with the aid of the examples drawn in the anatomy and the enquiries.) for the young, if numerous, are attached each to its umbilical cord, and this to the blood-vessel of the mother; they are arranged next to one another along the stream of the blood-vessel as along a canal; and each embryo is enclosed in its membranes and chorion.

those who say that children are nourished in the uterus by sucking some lump of flesh or other are mistaken. if so, the same would have been the case with other animals, but as it is we do not find this (and this can easily be observed by dissection). secondly, all embryos alike, whether of creatures that fly or swim or walk, are surrounded by fine membranes separating them from the uterus and from the fluids which are formed in it; but neither in these themselves is there anything of the kind, nor is it possible for the embryo to take nourishment by means of any of them. thirdly, it is plain that all creatures developed in eggs grow when separated from the uterus.

natural intercourse takes place between animals of the same kind. however, those also unite whose nature is near akin and whose form is not very different, if their size is much the same and if the periods of gestation are equal. in other animals such cases are rare, but they occur with dogs and foxes and wolves; the indian dogs also spring from the union of a dog with some wild dog-like animal. a similar thing has been seen to take place in those birds that are amative, as partridges and hens. among birds of prey hawks of different form are thought to unite, and the same applies to some other birds. nothing worth mentioning has been observed in the inhabitants of the sea, but the so-called ‘rhinobates’ especially is thought to spring from the union of the ‘rhini’ and ‘batus’. and the proverb about libya, that ‘libya is always producing something new’, is said to have originated from animals of different species uniting with one another in that country, for it is said that because of the want of water all meet at the few places where springs are to be found, and that even different kinds unite in consequence.

of the animals that arise from such union all except mules are found to copulate again with each other and to be able to produce young of both sexes, but mules alone are sterile, for they do not generate by union with one another or with other animals. the problem why any individual, whether male or female, is sterile is a general one, for some men and women are sterile, and so are other animals in their several kinds, as horses and sheep. but this kind, of mules, is universally so. the causes of sterility in other animals are several. both men and women are sterile from birth when the parts useful for union are imperfect, so that men never grow a beard but remain like eunuchs, and women do not attain puberty; the same thing may befall others as their years advance, sometimes on account of the body being too well nourished (for men who are in too good condition and women who are too fat the seminal secretion is taken up into the body, and the former have no semen, the latter no catamenia); at other times by reason of sickness men emit the semen in a cold and liquid state, and the discharges of women are bad and full of morbid secretions. often, too, in both sexes this state is caused by injuries in the parts and regions contributory to copulation. some such cases are curable, others incurable, but the subjects especially remain sterile if anything of the sort has happened in the first formation of the parts in the embryo, for then are produced women of a masculine and men of a feminine appearance, and in the former the catamenia do not occur, in the latter the semen is thin and cold. hence it is with good reason that the semen of men is tested in water to find out if it is infertile, for that which is thin and cold is quickly spread out on the surface, but the fertile sinks to the bottom, for that which is well concocted is hot indeed, but that which is firm and thick is well concocted. they test women by pessaries to see if the smells thereof permeate from below upwards to the breath from the mouth and by colours smeared upon the eyes to see if they colour the saliva. if these results do not follow it is a sign that the passages of the body, through which the catamenia are secreted, are clogged and closed. for the region about the eyes is, of all the head, that most nearly connected with the generative secretions; a proof of this is that it alone is visibly changed in sexual intercourse, and those who indulge too much in this are seen to have their eyes sunken in. the reason is that the nature of the semen is similar to that of the brain, for the material of it is watery (the heat being acquired later). and the seminal purgations are from the region of the diaphragm, for the first principle of nature is there, so that the movements from the pudenda are communicated to the chest, and the smells from the chest are perceived through the respiration.