"For each chromosome contributed by the sperm there is a corresponding chromosome contributed by the egg, there are two chromosomes of each kind, which together constitute a pair"

This quote by Thomas Hunt Morgan is referring to the procedure of fertilization. During fertilization, the sperm and egg each contribute chromosomes to the zygote. The sperm contributes one set of chromosomes, while the egg contributes another set. Each set of chromosomes is made up of 2 chromosomes, forming a pair. This set of chromosomes is known as a homologous set, and it is responsible for the hereditary information that is passed on to the offspring. This quote is stressing the significance of the homologous set in the fertilization procedure, as it is the mix of the 2 sets of chromosomes that figures out the genetic makeup of the offspring. Without the homologous pair, the offspring would not be able to acquire the hereditary details essential for its advancement.