Mishna: An egg which is laid on Yom Tov: Beis Shammai: It may be eaten. Beis Hillel: It may not be eaten.
Rav Yitzchok: The egg is forbidden because of gezeiras mashkin shezavu.
Rabbi Yochanan also explained the Mishna in terms of a gezeiras mashkin shezavu [like Rav Yitzchok]. This is implied by the question and answer which Rabbi Yochanan said in explaining Rabbi Yehuda’s position, as shall be explained:
Mishna: One may not squeeze fruits for juice on Shabbos or Yom Tov, if juice flowed from grapes and olives without being squeezed, the juices are forbidden.
Rabbi Yehuda: If they were prepared for food, their juice is permitted, if they were prepared for juicing, the juice is forbidden.
Rabbi Yochanan: So we see that according to Rabbi Yehuda, anything that comes from something prepared for food is consider like “a small piece of bread that was removed from a loaf”.
Mishna: More from Rabbi Yehuda [on the subject of two days of Rosh Hashana]: One can eat tevel (food from which terumah and maaser has not yet been separated) by making a condition [he separates it on the first day on condition it is a weekday, and then again on the second day, he can then eat it, since it was definitely separated on a weekday]. Similarly, an egg laid on the first day of Yom Tov may be eaten on the second day.
Rabbi Yochanan: It is only permitted on the second day but it is forbidden on the first day! Shouldn’t it be considered like “a small piece of bread that was removed from a loaf” - just like juice?
Rabbi Yochanan: We must invert the opinions in the first Mishna: The Chachomim differentiate between fruits prepared for food or for juicing, while Rabbi Yehuda always forbids it.
We see from Rabbi Yochanan’s question that in his understanding, an egg is forbidden because of gezeiras mashkin shezavu.