翌日、新聞社で、彼は中にダックスフントの入ったパンを描いた。ダックスフント・ソーセージではなく、犬のダックスフントの入ったものをである。

、、。・、。

The next day at the newspaper office, he drew a bread roll with a dachshund inside. Not a dachshund sausage — a [real] dog, a dachshund inside.

Literal

Next-day, newspaper-company [at-で], he [topic-は] inside [to-に] dachshund [subject-が] entered [attributive-の] bread [object-を] drew. Dachshund sausage is-not, dog [possessive-の] dachshund [subject-が] entered [attributive-の] thing [object-を] [emphatic-である].

A two-sentence joke about the etymology of 'hotdog.' The pun hinges on ダックスフント referring ambiguously to the sausage or the dog. 中にダックスフントの入ったパン is a complex relative clause: '[bread] into which a dachshund has entered.' 入った describes a resultant state. The である ending is formal/emphatic copula.