Atom is a plum pudding

The plum pudding model, also known as the blueberry muffin model, of the atom by J. J. Thomson, was proposed in $1904.$ In $1897$ Thomson showed through the cathode ray experiment that cathode rays were composed of a previously unknown negatively charged particles. The above left is the Thomson's cathode ray tube with electromagnetic deflection coils. In this model, the atom is composed of negatively charged particles surrounded by a soup of positive charge to balance the negatively charged particles, like negatively charged "raisins" surrounded by positively charged "pudding" as above right. What are the negatively charged particles?