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.
This is a necessary consequence of the fusion of two nuclei in fertilization, unless the chromosomes are to be doubled at each generation.
The nuclei of the original cells persist in the protoplasmic membrane.