Chromosomes were seen for the first time by

In the given diagram,$I$ and $II$ indicate:

Which of the following statements is/are correct?
$(a)$ The nuclear matrix or nucleoplasm contains the nucleolus and chromatin.
$(b)$ The outer nuclear membrane $(Ectokaryotheca)$ is usually continuous with the endoplasmic reticulum and also bears ribosomes on it.
$(c)$ Cells that are actively synthesizing protein have small and fewer nucleoli.

When the centromere is situated in the middle of the two equal arms of a chromosome,it is called:

Nucleolus is formed by

Which of the following is true for the nucleolus?