As before, only true ring nuclei, and not internal anhydrides of aliphatic compounds, will be mentioned.
Nuclei are everywhere present.
Here we meet with a great diversity of types: oxygen, nitrogen, sulphur and other elements may, in addition to carbon, combine together in a great number of arrangements to form cyclic nuclei, which exhibit characters closely resembling open-chain compounds in so far as they yield substitution derivatives, and behave as compound radicals.
Here structures have been observed which resemble nuclei in some of their characteristics.
This is a necessary consequence of the fusion of two nuclei in fertilization, unless the chromosomes are to be doubled at each generation.