The sac of reproductive cells in yeasts and other fungi is called an ascus.

Here's why:

* Asci are specialized cells within the fungal life cycle.

* They are formed during sexual reproduction.

* Inside the ascus, meiosis occurs, producing haploid spores called ascospores.

* These ascospores are the fungal reproductive cells, capable of germinating and developing into new fungal organisms.

The shape and number of ascospores within an ascus can vary depending on the fungal species.