Mishnah Niddah 6:1: If the lower sign [of two hairs] appears before the upper sign [of the breasts] does, she can participate in chalitzah [the ceremony performed to release a widow of a childless man from the obligation of levirate marriage] or yibum [levirate marriage]. If the upper sign appears before the lower sign does, even though this is impossible, Rabbi Meir says: she does not participate in chalitzah or yibum. And the Sages say: she must participate in either chalitzah or yibum. For they have said: it is possible for the lower sign to appear before the upper sign has, but it is impossible for the upper sign to appear before the lower sign has.

בָּא סִימָן הַתַּחְתּוֹן עַד שֶׁלֹּא בָא הָעֶלְיוֹן, אוֹ חוֹלֶצֶת אוֹ מִתְיַּבֶּמֶת. בָּא הָעֶלְיוֹן עַד שֶׁלֹּא בָא הַתַּחְתּוֹן, אַף עַל פִּי שֶׁאִי אֶפְשָׁר, רַבִּי מֵאִיר אוֹמֵר, לֹא חוֹלֶצֶת וְלֹא מִתְיַבֶּמֶת. וַחֲכָמִים אוֹמְרִים, אוֹ חוֹלֶצֶת אוֹ מִתְיַבֶּמֶת. מִפְּנֵי שֶׁאָמְרוּ, אֶפְשָׁר לַתַּחְתּוֹן לָבֹא עַד שֶׁלֹּא בָא הָעֶלְיוֹן, אֲבָל אִי אֶפְשָׁר לָעֶלְיוֹן לָבֹא עַד שֶׁלֹּא בָא הַתַּחְתּוֹן:

Mishnah Niddah 6:2: Similarly, any earthenware vessel [with a hole] that lets [liquids] in will also let [liquids] out, but there is [such a vessel] that let [liquids] out but does not let [liquids] in. Any limb that has a nail also contains a bone [and is therefore considered a limb such that it renders the impurity of a corpse, even if it has less than the required measure of dead flesh to be able to render impurity], but there may be [a limb] that contains a bone but does not have a nail [and which would not be considered a limb for the purpose of rendering the impurity of a corpse, unless it contains the required measure of flesh to be able to render impurity].

כַּיּוֹצֵא בוֹ, כָּל כְּלִי חֶרֶס שֶׁהוּא מַכְנִיס, מוֹצִיא. וְיֵשׁ שֶׁמּוֹצִיא וְאֵינוֹ מַכְנִיס. כָּל אֵבָר שֶׁיֶּשׁ בּוֹ צִפֹּרֶן, יֶשׁ בּוֹ עֶצֶם, וְיֵשׁ שֶׁיֶּשׁ בּוֹ עֶצֶם וְאֵין בּוֹ צִפֹּרֶן: