We are in chapter 2, mishnah 9. Rabbi Akiva comes here to teach something that is essentially aggadah rather than halachah, and in it he establishes a principle in the ways in which the Holy One, Blessed is He, bestows good upon a person and upon his descendants.
Rabbi Akiva's words in the mishnah:
He used to say: "Ha'av zocheh laben b'noy, uv'koach, uv'osher, uv'chochmah, uv'shanim" - a father confers upon his son five things:
"B'noy" - in beauty.
"Uv'koach" - in strength.
"Uv'osher" - in wealth.
"Uv'chochmah" - in wisdom.
"Uv'shanim" - in length of days.
Two approaches to understanding this bestowal:
Some explain, and this is the approach of the Bartenura, that we are dealing here with a natural phenomenon: the father benefits his son by his own power - through heredity, through the legacy he leaves and through everything that passes from him to his son, and by that power he grants him these things.
Another approach, also brought in the Bartenura (and in the words of the Rishonim): a person has merits on account of his good deeds, and because of them the Holy One, Blessed is He, rewards him by granting these blessings not necessarily to the father himself, but to his son.
It is quite possible that both approaches are true.
"Umispar hadoros lefanav, v'hu haketz":
At times the Holy One, Blessed is He, commits Himself to bestow good upon a person's descendants, and He even specifies, as it were, a time frame for it. But ultimately the commitment is measured in generations and not in years - in his son, his grandson or his great-grandson - and not in a particular number of years. The count of generations is the point of conclusion, it is "haketz."
The example from the Bris Bein HaBesarim:
The mishnah brings an explicit example of this: the Holy One, Blessed is He, told Avraham at the Bris Bein HaBesarim that He would bestow good upon his descendants, that they would be enslaved for four hundred years, and that in the fourth generation they would return to Eretz Yisrael. In practice we know that the stay in Egypt did not last four hundred years but two hundred and ten, and even during those years they were enslaved only for part of the time. Although the Holy One, Blessed is He, arranged matters in such a way that the count of four hundred works out, yet on the face of it that count is not precise in its plain sense, while the timetable that was fulfilled exactly was the number of generations - four generations.
And this is the count of the generations:
Kehas ben Levi, who went down to Egypt.
Amram his son, who was born in Egypt.
Moshe Rabbeinu, the son of Amram.
The sons of Moshe, who had already returned to Eretz Yisrael.
The Mishnah anchors this principle in the verse: "Korei hadoros meirosh" - the Holy One, blessed is He, as it were, counts and evaluates the generations from the very beginning of creation, for since He knows all, He knows in advance how events will unfold over the course of history.
The wording of our Mishnah is "V'af al pi shene'emar" - "and even though it says", but there are other versions that read "v'ne'emar" or simply "shene'emar". The verse the Mishnah brings is: "Va'avadum v'inu osam arba me'os shanah" - they will be in servitude and affliction for four hundred years.
Yet the passage continues: "V'dor revi'i yashuvu heinah" - and the fourth generation will return here. Note the contrast: the count of four hundred years requires calculation and adjustment in order to align with the reckonings of the Holy One, blessed is He, according to the condition of Israel, their needs and what they were worthy of, whereas the fourth generation did in fact return to Eretz Yisrael exactly as promised.
There is also another count of four generations, from Yehudah until Kalev ben Yefuneh, and it expresses the very same idea: four generations, just as the Holy One, blessed is He, committed from the outset, and so it actually came about.
In summary: In this Mishnah, Rabbi Akiva taught us that a father grants his son beauty, strength, wealth, wisdom and years - both by way of nature and heredity and by way of his own merits. We saw that this benefit is measured by "the number of generations before him", as we observed at the Covenant Between the Parts: the count of four hundred years required calculation, while the count of four generations was fulfilled in its plain sense. It is the passing generations, and not the years in themselves, that are the primary factor in the goodness the Holy One, blessed is He, bestows upon a person's descendants.