Examples from the Web for egg sac
One of the running spiders (Dolomedes) carries her egg-sac with her mandibles until the young are ready to emerge.
These eggs hatch early in the winter; but no spiders emerge from the egg-sac until the following spring.
In a spider's egg-sac the young after hatching do not immediately leave the sac, but remain in it for several days.Elementary Zoology, Second Edition|Vernon L. Kellogg
In the specimen figured, the egg-sac was concealed in the upper part of the web.