While discussing the status of a miscarriage for the purposes of determining whether or not the mother becomes ritually unclean as a woman who gave birth (tum’at leidah), the Mishna on daf 21a brought a disagreement between Rabbi Meir and the Sages:
If an abortion had the shape of a beast, a wild animal or a bird, whether clean or unclean, if it was a male she must continue in uncleanness and subsequent cleanness for the periods prescribed for a male, and if it was a female she must continue in uncleanness and subsequent cleanness for the periods prescribed for a female, but if the sex is unknown she must continue in uncleanness and subsequent cleanness for the periods prescribed for both male and female; so Rabbi Meir. The Sages, however, ruled: anything that has not the shape of a human being cannot be regarded as a human child.
On today’s daf, a number of opinions are brought in an attempt to clarify the source of Rabbi Meir’s position.
Rabbi Hiyya bar Abba quotes Rabbi Yohanan as teaching that in the language of creation in Sefer the term yetzirah – to form or to create – appears regarding both animals (2:19) and people (2:7).
Another suggestion made by Rabbi Hiyya bar Abba in the name of Rabbi Yohanan is that their eyes are similar.
Rabbi Yannai suggests that the eyes of animals are fixed forward, like those of people.
According to Rabbi Meir who ruled: ‘A beast that was in a woman’s body is a valid birth,’ what is the law where its father received a token of betrothal on its behalf?
In what respect could this ever matter? — In respect of causing its sister to be forbidden. This then presumes that it is viable! But did not Rav Yehuda citing Rav state: Rabbi Meir gave his ruling only because in the case of its own species it is viable?Said Rav Aha bar Ya’akov: ‘To such an extent did Rabbi Yirmeya try to make Rabbi Zeira laugh; but the latter did not laugh.’
Rashi explains that Rabbi Yirmeya certainly understood that the form of an animal developing as a human embryo is not viable and that accepting money for its betrothal has no significance in Jewish law, but he wanted to joke with Rabbi Zeira who was a very serious individual. The Arukh LaNer explains that although humor can be appropriate in the context of Torah study (see Tehillim 100:2), Rabbi Zeira was stringent and never allowed himself any amusement. Rabbi Yirmeya tried – unsuccessfully – to share with him a small joke.