On the Seder night, we break up our recitation of Hallel into two separate parts. The first part of Hallel, meaning the first two paragraphs, is said at the end of Maggid, prior to the meal. The remainder of Hallel is not recited until the end of the Seder, after the entire meal and Birchas HaMazon. An interesting question may be asked about this practice of dividing Hallel. The Gemara in Berachos (דף י"ד.) discusses whether we are ever permitted to interrupt our recitation of Hallel; clearly, it is preferable to avoid such interruptions. How then can we break up our recitation of Hallel at the Seder with the meal, and various other activities that seem to have nothing to do with Hallel, which would ordinarily be considered an inappropriate interruption?
One possible answer to this, suggested by the Netziv, is that eating is in fact a form of Hallel. When we eat, especially at the Seder when we thank Hashem for all that He has done for us, we are praising Hashem in a different fashion. A proof for this idea may be found in a Mishnah in Taanis (דף י"ט.). The Mishnah is discussing the procedure of fasting and davening in Shul because of a lack of rain, and states that it once happened that people were fasting and davening when it suddenly started to rain. Rabbi Tarfon told the people to go home and eat and drink and celebrate, which they did, and they then came back to Shul and recited Hallel. The obvious question is why they had to go home and eat before they said Hallel; they were all already in Shul, so why couldn't they say Hallel before they went home? The answer is that eating and drinking is also a way to thank Hashem, as is the recitation of Hallel. Rabbi Tarfon evidently felt that it was important for the people to recognize that they can praise Hashem even by eating and drinking before they would recite the formal Hallel. Similarly, on Pesach night, we must recognize that we can praise Hashem in many ways, including by eating and drinking; the meal at the Seder is thus not an interruption within Hallel, but is in fact a part of Hallel.