第三步
這兩個次級精母細胞也移動，更靠近生精小管的中央，各自按照普通的方式再分裂，精確地複製出了它們自身。這4個細胞被稱為精子細胞。現在，精子細胞變形，發育出尾，因而發育成成熟的、稱之為精子的細胞。經過這三個發育階段的完整過程，需要約64天。從精子的起源和發育就可明瞭，它分成了兩類細胞：含有X染色體（22條常染色體）的精子和含有Y染色體（22條常染色體）的精子。一旦授精，與X染色體關聯的精子會協助產生女孩；與Y染色體關聯的精子會協助產生男孩。

生精小管內部觀

1.精原細胞；2.初級精母細胞；3.次級精母細胞；4.精子細胞；5.細胞分裂；6.間質細胞產生睾丸激素；7精子。

Inside a seminiferous tubule.
1. spermatogonium 2. primary spermatocyte 3. secondary spermatocyte
4. spermatids 5. cell division 6. interstitial cells producing testosterone 7. sperm cells

The Third Step
The two secondary spermatocytes move even closer to the center of the tube, and each divides again in the ordinary fashion, duplicating itself exactly. The four new cells are called spermatids. These spermatids now change their shape, develop a tail, and thus grow into mature sperm cells called spermatozoa. The entire process through all three stages of development takes about 64 days. As is obvious from their origin and development, spermatozoa come in two varieties: those containing an X chromosome (and 22 other chromosomes), and those containing a Y chromosome (and 22 other chromosomes). In case of a fertilization the X-bearing spermatozoa will help to produce girls; the Y-bearing spermatozoa will help to produce boys.