Final answer:
The exact age in days for a bovine fetus the size of a rat cannot be determined with the provided information. The gestation period for cattle is about 283 days, and a rat-sized fetus would likely be in the second trimester of gestation.
Step-by-step explanation:
When a bovine fetus is the size of a rat, it corresponds to a specific stage of its gestation period. However, to determine the exact age in days, we must consider that gestation varies substantially among different species, and the gestation length for cattle (bovines) is typically around 283 days on average. While information provided centers on human fetal development and gestation lengths for various animals, the bovine gestation period is not directly discussed. That said, a rat-sized bovine fetus could likely be within the second trimester, as fetal growth is rapid during this period. However, the given data does not provide enough specificity to accurately pinpoint the fetal age in days for a bovine fetus of this size without additional information on bovine fetal development stages.