"Bhikkhus, the decent of the embryo takes place through the union of three things..."
BB: The following portion of the discourse may be understood as the concrete application of dependent origination --- so far expressed only as a doctrinal formula --- to the course of individual existence.
The factors from consciousness through feeling result from past ignorance and formations, the cause factors of craving and clinging build up a continuation of the samsaric round, and finally dependent origination is connected to the appearance of the Buddha and his teaching of the Dhamma, showing that the practice of the Dhamma is the means of bringing the round to and end. "... when there is the union of the mother and the father, and the mother is in season, and the gandhabba is present, though the union of these three things the descent of the embryo takes place." MA:
the gandhabba is the being arriving there. It is not someone (i.e. a disembodied spirit) standing nearby watching the future parents having intercourse, but a being driven on by the mechanism of kamma, due to be reborn on that occasion. BB:
The exact import of the word grandhabba in relation to the rebirth process is not explained in the Nikayas, and the word in this sense occurs only here an in DN 15 http://www.accesstoinsight.org/tipitaka ... .than.html
DN 15 speaks of conciousness as "descending into the mother's womb", this being a condition for rebirth to take place. thus we may identify the gandhabba here as the stream of conscisousness, conceived mroe animistically as coming over from the previous existence and bringing along its total accumulation of kammic tendencies and personality traits. The fullest study of the concept of the gandhabba is Vijesekera "Vedic Gandharva and Pali Gandhabba", in Buddhist and Vedic Studies, pp. 191-202.