רבינו בחיי — אשה כי תזריע

Q — The gemara teaches "אשה מזרעת תחלה יולדת זכר" — but mechanistically, why should the order matter?

A — Rabbeinu Bachya supplies the underlying picture and it is conceptually beautiful. Both seeds are *real* contributions, but they play asymmetric roles set by sequence: the first deposit is the field (קרקע, the ground in which something will be sown), and the second deposit is the seed (זרע, that which goes into the ground). Whichever seed arrives *second* is therefore the determinative one — the upper hand, דעילאה גבר. If the wife is mazri'ah first, her seed becomes the field and the husband's later seed is the actual zera, and the child takes the form of that planted seed: zachar. If the husband is mazri'a first, the inversion holds and the child is nekeivah. RBB layers a moral consequence on this: a man who is mash'heh atzmo al ha'beten — who patiently waits so that his wife is mazra'as first — earns שכר פרי הבטן, the reward of sons (Tehillim 127:3, as the gemara reads it). The vayered chesed and patience produce zerah zechor.