A given receptor is described based on each classification criterion: receptor distribution, stimulus origin, and modality of stimulus. The eyes, for example, are because they are located in the head (receptor distribution); because they detect stimuli outside the body (stimulus origin); and because they detect light (modality of stimulus). In comparison, receptors that detect stretch of blood vessels are classified as because they are distributed throughout the body, because they detect stimuli within the body, and because they detect changes in distension of the organ wall.

fertilization, pictured in figure 24.23a is the process in which gametes (an egg and sperm) fuse to form a zygote. the egg and sperm each contain one set of chromosomes. to ensure that the offspring has only one complete diploid set of chromosomes, only one sperm must fuse with one egg. in mammals, the egg is protected by a layer of extracellular matrix consisting mainly of glycoproteins called the zona pellucida. when a sperm binds to the zona pellucida, a series of biochemical events, called the acrosomal reactions, take place. in placental mammals, the acrosome contains digestive enzymes that initiate the degradation of the glycoprotein matrix protecting the egg and allowing the sperm plasma membrane to fuse with the egg plasma membrane, as illustrated in figure 24.23b. the fusion of these two membranes creates an opening through which the sperm nucleus is transferred into the ovum. the nuclear membranes of the egg and sperm break down and the two haploid genomes condense to form a diploid genome.

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