Nutrix ejus terra est.
His nurse (nutrix ejus) is the earth (terra est).
Note that this is nurse not in the sense of doctor-and-nurse but in the sense of a nurse-maid, someone who nourishes (nutrix/nutrition) a baby.
Romulus hirta lupae pressisse, sed ubera caprae
Jupiter, & factis, fertur adesse fides:
Quid mirum, tenerae SAPIENTUM viscera PROLIS
Si ferimus TERRAM lacte nutrisse suo?
Parvula si tantos Heroas bestia pavit,
QUANTUS, cui NUTRIX TERREUS ORBIS, erit?
[I have taken it upon myself to correct "tantas Heroas" to "tantos Heroas" so that the noun and adjective agree; Heroas is a Greek form, but masculine in gender.]
Romulus is said to have squeezed (Romulus pressisse fertur) the shaggy teats of a she-wolf (hirta lupae ubera) but Jupiter those of a nanny-goat (sed caprae Jupiter), and faith is said to have been put in those events (et factis adesse fides). What's odd then (quid mirum) if we report (si ferimus) that the EARTH nourished with her milk (TERRAM lacte nutrisse suo) the flesh of the tender OFFSPRING (tenerae viscera PROLIS) of the WISE MEN (SAPIENTUM)? If so small a beast (parvula si bestia) fed such great Heroes (tantos Heroas pavit), HOW GREAT will be he (QUANTUS erit) whose NURSE (cui NUTRIX) is the EARTHLY ORB (TERREUS ORBIS)?
You can read about Romulus and his twin Remus and the wolf who suckled them at Wikipedia: Capitoline Wolf.
The goat who suckled Jupiter also has an article at Wikipedia: Amalthea. She is the origin of the famous aegis and also of the constellation Capra (but not Capricorn; that is a differently mythological creature). Amalthea's horn is the proverbial "cornucopia," the horn (cornu) of plenty (copia, as in the English word copious).
Finally, I am so struck by the depiction of the mythological Earth-Nurse here because it reminds me of the depiction of Bhu-Devi, the Earth-Goddess, mother of Sita, in Nina Paley's brilliant film "Sita Sings the Blues." The opening of the film shows a great cosmic dance where you can see Paley's version of Bhu-Devi at about 4:45 into the opening sequence:
And you can see Nina Paley's Bhu-Devi in this poster: the colossal size here definitely matches the sense of scale that Michael Maier wants to convey in his Atalanta fugiens emblem:
And here is a list of the emblems I have completed so far:
Emblem I: Portavit eum ventus in ventre suo.
Emblem II: Nutrius ejus terra est.
Emblem XXXVI: Lapis projectus