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 leida), the Mishna on daf 21a brought a disagreement between Rabbi Meir and the Sages:
With regard to a woman who discharges tissue in the form of a type of domesticated animal, undomesticated animal or a bird, whether it had the form of a non-kosher species or a kosher species, if it was a male fetus then she observes the periods of impurity and purity for a woman who gives birth to a male, and if the fetus was a female the woman observes the periods of impurity and purity established for a woman who gives birth to a female, but if the sex is unknown she observes the strictures that apply to a woman who gave birth to both a male and a female; this is the statement of Rabbi Meir. And the Rabbis say: Any fetus that is not of human form is not regarded as an offspring with regard to observance of these periods.
On today’s daf, a number of opinions are brought in an attempt to clarify the source of Rabbi Meir’s position.
Rabbi Ḥiyya bar Abba quotes Rabbi Yoḥanan as teaching that in the language of creation in Sefer the term yetzira – to form or to create – appears regarding both animals (2:19) and people (2:7).
Rabba bar bar Ḥana says in the name of Rabbi Yoḥanan 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 said that an animal in the womb of a woman is considered a full-fledged offspring, what is the law in a case where it is a female and her father accepted betrothal for her? Is such a betrothal valid?
What practical difference is there whether it is valid? — with regard to whether it is prohibited for the man to marry her sister. This then presumes that it is viable! But did not Rav Yehuda citing Rav state: Rabbi Meir gave his ruling only because there are other animals of its type that can live. 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.