The Supreme Personality of Godhead chooses a mother and father from whom to take birth again and again. The Lord took birth originally from Sutapa and Prsni, then from Kasyapa and Aditi, and again from the same father and mother, Vasudeva and Devaki. In other appearances also, the Lord said, I took the form of an ordinary child just to become your son so that we could reciprocate eternal love. Jiva Gosvami has explained this verse in his Krsna-sandarbha, Ninety-sixth Chapter, where he notes that in text 37 the Lord says, amuna vapusa, meaning by this same form. In other words, the Lord told Devaki, This time I have appeared in My original form as Sri Krsna. Srila Jiva Gosvami says that the other forms were partial expansions of the Lords original form, but because of the intense love developed by Prsni and Sutapa, the Lord appeared from Devaki and Vasudeva in His full opulence as Sri Krsna. In this verse the Lord confirms, I am the same Supreme Personality of Godhead, but I appear in full opulence as Sri Krsna. This is the purport of the words tenaiva vapusa. When the Lord mentioned the birth of Prsnigarbha, He did not say tenaiva vapusa, but He assured Devaki that in the third birth the Supreme Personality of Godhead Krsna had appeared, not His partial expansion. Prsnigarbha and Vamana were partial expansions of Krsna, but in this third birth Krsna Himself appeared. This is the explanation given in Sri Krsna-sandarbha by Srila Jiva Gosvami.