The links of St Thomas's argument are these:
(a) The Word was made flesh the very instant that His Humanity was conceived, the very instant that Mary spoke the word: Be it done to me according to thy word.
(b) The Word would not take flesh otherwise than by assuming a body there and then animated with a rational soul.
(c) A rational soul cannot inform a body not yet developed to human shape. In the ordinary course of human embryonic development, the embryo at conception, being incapable even of a sentient, still more of a rational soul, is animated with a vegetative soul, which after some days gives place to a sentient soul, and that after more days are expired, and the foetus is come to human shape, is finally replaced by a rational soul: all which process is drawn out at length in B. II, Chapp. LXXXVIII, LXXXIX.
(d) This ordinary process of nature had to be set aside in the formation of Mary's miraculous Child. His Body was complete from the first, a fit receptacle for a rational soul. His Body consequently did not develop, it simply grew.
Now the link (c) of this chain is broken by modern Catholic theologians. They see no difficulty in a rational soul informing a body not yet developed to human shape. They hold that the rational soul is always infused in the very instant of conception. Thereupon they conclude that the way of formation of Christ's body, after conception, in no way differed from that of other human bodies, nihil differens fuisse a reliquis foetibus humanis (Pesch, Praelectiones Dogmaticae, vol. IV, p. 85, ed. 1896).
Scripture is silent on the subject; modern biology would be amazed at such a mode of growth as St Thomas and Suarez after him suppose; and miracles, as Suarez himself here owns, are not to be multiplied without necessity or high congruity (Suarez, De mysteriis Christi, disp. II, sect. 2, nn. 2, 4).
This discussion has an extrinsic interest as illustrating two several views of another mighty development, that of Church government and doctrine. The development of the Bridegroom may well be the pattern of that of the Bride.
Accepting St Thomas's supposition of the three successive souls, as a supposition not yet quite exploded, there is still some doubt as to his conclusion, in point of link (b). The Word remained united with the dead Body of Christ, from whence all soul was departed: might it not then unite itself with a living Body into which in due course of nature a rational soul was soon to come?
Of God and His Creatures: 4.44