In Generation of Animals, he finds a fertilized hen's egg of a suitable stage and opens it to see the embryo's heart beating inside.
ɪn dʒˌɛnɚɹˈeɪʃən ʌv ˈænɪməlz, hiː fˈaɪndz ɐ fˈɜːɾɪlˌaɪzd hˈɛnz ˈɛɡ əvə sˈuːɾəbəl stˈeɪdʒ ænd ˈoʊpənz ɪt tə sˈiː ðɪ ˈɛmbɹɪˌoʊz hˈɑːɹt bˈiːɾɪŋ ɪnsˈaɪd.