Bartholomew says: "The infant is generated from seeds which have contrary qualities; it is located on the right side if it is male, and on the left if it is female. It is fed and warmed by menstrual blood in the uterus. Man receives his nutrition from such common and weak material from the beginning, by the power of heat; and as nature unites and extends the limbs one by one, little by little, he is formed - but not immediately, for only Christ was formed immediately and separately in the uterus, since he was conceived in the uterus, as Augustine says. When the soul enters, life is poured forth; perceiving by natural sense the surrounding of the membrane, it is moved to break it, and the mother's uterus is agitated and made heavy by this motion. When nature's activity for the creation of a baby is complete, if the parts are healthy then in the eighth, ninth, or tenth month it struggles and strives to leave the uterus."

Original Latin

No microscope - No problem! We've got authorities
Aristotle reports observations of embryos, but detailed examination was impossible, since the microscope was not invented until 1590. Not surprisingly Bartholomew generally relies on authorities. In this text, he cites

• Aristotle (384 BCE to 322 BCE), whose texts of natural philosophy became available in the early 13th century, not long before Bartholomew's time
• Augustine of Hippo (354 CE to 430 CE)
  
• Constantine the African (ca. 1020 CE to 1087 CE), whose whose knowledge of Arabic and Greek greatly expanded medical learning in medieval Europe.

Fetal Development
Bartholomew reports a traditional scheme for explaining prenatal development: “Seminal humor” (or "special blood," the text is not clear) from both parents is necessary to have a baby, in part because both have different properties that have to be balanced. At first the humors are spread out but then they gather together and begin to ferment, as yeast ferments bread dough. If this gathering takes place in the right side of the womb, a boy is produced. If in the left, then a girl.

Probably Bartholomew owes this idea to Aristotle, who says that a baby's first movement occurs on the right side of the womb if the infant is a male and on the left if a female (History of Animals 7.3.583b3). Similarly, the tale of infants who have to crawl rather than walk on account of being top heavy comes from Aristotle's Parts of Animals, book 4, chapter 10.

Other Interesting Medieval Ideas
Other medieval sources make various changes to this simple scheme. The most notable one may be the idea that the womb had seven cells or receptacles inside it: three on the left, three on the right, and one in the middle. This conveniently explained how there might be twins or even triplets, because then the humors could gather in the various cells and develop independently into multiple fetuses. The one in the middle was of course where hermaphrodites developed.

An anonymous verse helped people remember the stages of fetal development.

Sex in lacte dies, ter sunt in sanguine terni,
Bis seni carnem, ter seni membra figurant.

Six days in milk, thrice three are in blood,
Twice six flesh, thrice six in members are figured.1

But that adds up to only 45 days in the womb, which we know isn't right (and presumably, empirical evidence would have proved this verse wrong as well!):

Here, Bartholomew is working in a different tradition and is not concerned with the rest of the time, evidently before these developments take place and especially afterwards, to allow time for growth. The number 46 is significant in the Christian tradition, since it took 46 years to build the Temple in Jerusalem, according to Augustine, to which may be compared the perfection of the body of Christ.

Thus, Bartholomew would like the stages to add up to be 46 as well, so he fudges a bit to emphasize Augustine's moral point. Furthermore, when 46 is multiplied by 6, we get 276, or 9 months and 6 days: 276 = (30 x 9) + 6, a number much closer to that required for human gestation.

Thus, indeed, Christ was born on the 8th kalends of January, 276 days after the 8th kalends of April. More about calculating Christ's birthday

Advice Both Sensible and Strange
Do you have younger siblings? Do you remember the sorts of things your parents did to comfort them when they were infants? You might recognize some of those same things in Bartholomew's descriptions of how to care for a newborn, although you will immediately notice the combination of sensible advise with strange theories Bartholomew offers his readers.

On the sensible side: even now doctors advise new parents to rub the palates of infants - today we are told that it is to stimulate the sucking response. Sometimes they also advise swaddling newborns. And everyone agrees that the diet of the nursing mother (or her substitute in former times, the wet nurse) is important for the health of the baby.

Some of the strange theories reported here are just odd, but others are the consequence of medieval chemical theory. For example, the belief that heat is the principal active force in digestion and growth and more generally in all chemical change.

Advising that babies rest in dark places may just be common sense - avoid overly bright lights that would startle a newborn. But suggesting a dark place "so that the light of their eyes can coalesce" probably reflects belief in an extramission theory of vision. Such theories indicate that the eye sees something not as a result of light striking the eye, but at least in part as a result of the eye's projecting a visual ray to the object seen.

1: The poem is a general bit of folk wisdom, quoted by other authors including Thomas Aquinas: [7906] Super Sent., lib. 3 d. 3 q. 5 a. 2 co.