Bekhorot 8A

הדולפנין פרין ורבין כבני אדם. מאי דולפנין? אמר רב יהודה: בני ימא.

The dulfanin reproduce like people. The Talmud asks: What are dulfanin? Rav Yehuda says: They are creatures that are called sons of the sea.

כל שביציו מבחוץ ⁠— מוליד, וכל שבפנים ⁠— מטיל ביצים.

The baraita continues: In the case of any male animal whose testicles are external, the female gives birth to live offspring, and in the case of any male animal whose testicles are internal, the female lays eggs.

איני? והאמר שמואל: אווז ואווז בר כלאים זה בזה, והוינן בה: מאי טעמא? אמר אביי: זה ביציו מבחוץ, וזה ביציו מבפנים, ותרוייהו מטילי ביצים!

The Talmud asks: Is that so? But doesn’t Shmuel say that a domestic goose and a wild goose are considered diverse kinds, and one may not mate them with each other. And we discussed it: What is the reason? Abaye said: In the case of this one, the male wild goose, its testicles are external, and in the case of that one, the domestic goose, its testicles are internal. The Talmud comments: And yet both geese lay eggs. Evidently, the fact that the male’s testicles are external does not prove that the female gives birth.

אלא, כל שזכרותו מבחוץ ⁠— מוליד, מבפנים ⁠— מטיל ביצים.

Rather, the baraita must mean the following: With regard to any animal whose male reproductive organ is external, the female gives birth, and in the case of any male animal whose reproductive organ is internal, the female lays eggs. Although the testicles of the male wild goose are external, its reproductive organ is internal.

כל שתשמישו ביום ⁠— יולד ביום, בלילה ⁠— יולד בלילה, כל שתשמישו בין ביום בין בלילה ⁠— יולד בין ביום בין בלילה.

§ The baraita continues to discuss matters of animal procreation: Any species whose sex occurs only during the daytime gives birth only during the daytime; any species whose sex occurs only at night gives birth only at night; any species whose sex occurs both during the daytime and at night gives birth both during the daytime and at night.