Commentary for Avodah Zarah 79:13
ואלו הן סימני ביצים כל שכודרת ועגולגלת ראשה אחד כד וראשה אחד חד טהורה ב' ראשיה חדין וב' ראשיה כדין טמאה חלמון מבחוץ וחלבון מבפנים טמאה חלבון מבחוץ וחלמון מבפנים טהור חלבון וחלמון מעורבין זה בזה זו היא ביצת השרץ אמר רבא כשנימוחו
Clean fish? Does this make sense? Scripture mentions fins and scales as the marks of [clean] fish! Rather the meaning is: The marks of [clean birds’] eggs are the same as those of fish-roe [which may be eaten]; and the following are the marks of [clean] birds’ eggs: Any one that narrows at the top and is rounded, one of its ends is wide and the other end is pointed is kosher. If both of its ends are sharp or both of its ends are wide, it is non-kosher. If the yolk is on the outside and the albumen is on the inside, it is non-kosher; if the albumen is on the outside and the yolk is on the inside, it is kosher. If the yolk and albumen are mixed with each other, this is the egg of a creeping animal. Rava said: [Rav's statement that it must only be bought from an expert refers to when the roe] has been pressed.