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The Evolution of Reproduction 
Multicellular organisms began to evolve and produce specialized cells; some also produced 
tissues and organs that performed specific roles. In the annelids, reproduction underwent an early 
development. These organisms create sperm and eggs in their coelom from undifferentiated cells and 
store them there. When the coelom is full, the body either splits open or an excretory opening allows 
the cells to exit. The development of gonads, which produce sperm and eggs, led to the evolution of 
reproductive organs. These cells underwent meiosis, an adaptation of mitosis that results in cell 
division while cutting the number of chromosomes in each reproductive cell in half. 
Complete reproductive systems were developed in insects, with separate sexes. Sperm are 
made in testes and then travel through coiled tubes to the epididymis for storage. Eggs mature in the 
ovary. When they are released from the ovary, they travel to the uterine tubes for fertilization. Some 
insects have a specialized sac, called a 
spermatheca
, which stores sperm for later use, sometimes up 
to a year. Fertilization can be timed with environmental or food conditions that are optimal for 
offspring survival. 
Vertebrates have similar structures, with a few differences. Non-mammals, such as birds and 
reptiles, have a common body opening, called a 
cloaca
, for the digestive, excretory and reproductive 
systems. In order to transfer sperm, birds typically couple by positioning their cloacal openings 
opposite one another. Mammals have uteruses that support developing young and separate openings 
for the female's reproductive systems. In species that give birth to many offspring at once, the uterus 
has two chambers, whereas species that only give birth to one offspring, like primates, have a single 
uterus. 
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