Made when sediments fill in a cavity made when an object decayed and the sediments harden into rock

A fossil is formed when a plant, animal or other organism dies. The soft tissues of these organisms gets decomposed and degraded by decomposers present in the soil. The remaining hard and undecayed part like wood, shells, skeleton are left. These remains are covered by layers of soils. These layers of soil gets compacted to produce rocks. The rocks exhibit the dead remains are called as cast. A cast is also termed as mould fossil. It is the general form of the original organism.  

Therefore, the description: Made when sediments fill in a cavity made when an object decayed and the sediments harden into rock is cast.

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.

explanation:

heat a spark and oxygen