How did Jan gain the access to the Ghetto, which allowed him to help the Jews escape as noted in the nonfiction book, The Zookeeper’s Wife?

Jan stored the extensive insect collection of Szymon Tenenbaum at the zoo. The insect collection became Jan’s way of gaining access to the Ghetto when a German officer came to see it after hearing about it from Szymon. Jan convinced the German that he needed to occasionally see Szymon, who was confined to the Ghetto, to discuss the care of the collection. After that, Jan was able to freely enter the Ghetto, so he used his access to help Jews escape.