"Hoopsa boyaboy hoopsa!": this third verse in the fertility chant imitates, according to Gifford, "The cry with which a midwife celebrates the birth of a male child as she bounces it to stabilize its breathing."
A newborn baby may need some help to start drawing its first breaths of air. Nowadays doctors quickly suction the airway to remove any fluid, but one traditional method was to lift the child by its ankles and slap it on the buttocks, using gravity to dislodge the fluid. Whatever the method being represented here, Joyce's "hoopsa" does seem to take us from ceremonially turning to the maternity hospital in the first tercet of the chant, and invoking the gods for fertility in the second, to the actual bouncing, breathing, screaming product of gestation and labor, here at the conclusion of the chant.
The progressive evolution of prose styles in Oxen of the Sun parallels the passage of time waiting for Mina Purefoy to give birth, and Joyce seems also to intend an analogy between the development of English prose and the development of a fetus. The schemas which he gave to Linati and Gilbert identify the "technique" of this chapter as "embryonic development."
How exactly one should understand the details of this analogy is a very uncertain business. Much ink has been spilled tying particular moments in the prose narration to particular moments in the gestational process. Of all such interpretations, Joyce's own is perhaps the goofiest: "Bloom is the spermatozoon, hospital the womb, the nurses the ovum, Stephen the embryo" (quoted in Gifford, 408).
Gifford's general summation of the significance of the analogy, however, seems very persuasive: "In effect, the sequence of imitations is a sustained metaphor for the process of gestation; Joyce would have assumed that in that process ontogeny (the development of the individual organism) recapitulates phylogeny (the evolutionary history of the species)—what Joyce called "the periods of faunal evolution in general" [Letters 1:140]); thus the development of the embryonic artist's prose style recapitulates the evolution of prose style in literary history." It is probably no coincidence that the novel's first reference to Charles Darwin comes in Oxen of the Sun.