Senior synonyms. The earlier published of synonyms stimuli.

Sense-organ. Receptor.

Sensory. Concerned with receptors. S. nerve. Peripheral nerve consisting of nerve fibres of sensory nerve cells. S. nerve fibre. Nerve fibre of whose fibre connect with receptor, transmitting impulses started by receptor to central nervous system. In vertebrate the cell body of such a nerve cell is attached to its nerve fibre at a point along its course situated in dorsal root ganglions, and there are no dendrites. In invertebrates the cell body is situated at (peripheral) end of nerve fibre furthest away from central nervous system. S. Root. Of vertebrates, nerve root containing the sensory nerve fibres.

Sere. A plant succession, a progression of plant communities with time.

Series. The sample available for taxonomic study.

Serous. Resembling or producing.

Sex chromosome. Chromosomes of which there is a homologous pair in the nuclei of one sex (the homogametic sex,) and a dissimilar pair (or an unpaired chromosome) in those of the other (the heterogametic sex). Sex chromosomes which occurs as a pair in the homogametic sex are called X-chromosomes. The heterogametic sex has an X-chromosome and either a Y-chromosome or none.
The homogametic (or XX) sex produces gametes which are identical in their sets of chromosomes, all containing one X-chromosomes. The heterogametic (XY or XO) sex produces an equal number of two different sorts of gametes, one with and one without an X-chromosomes. Union of the gametes of the XX sex with those of the XY (or XO) sex therefore results in offspring consisting of approximately equal numbers of the two sexes.
The presence or absence of a Y-chromosome determines sex in mammals; in insects of a Y-chromosome determines sex in mammals; in insects it is determined by the number of X-chromosomes in relation mechanism is of course absent in hermaphrodite animals and plants.