第三步
这两个次级精母细胞也移动，更靠近生精小管的中央，各自按照普通的方式再分裂，精确地复制出了它们自身。这4个细胞被称为精子细胞。现在，精子细胞变形，发育出尾，因而发育成成熟的、称之为精子的细胞。经过这三个发育阶段的完整过程，需要约64天。从精子的起源和发育就可明了，它分成了两类细胞：含有X染色体（22条常染色体）的精子和含有Y染色体（22条常染色体）的精子。一旦授精，与X染色体关联的精子会协助产生女孩；与Y染色体关联的精子会协助产生男孩。

生精小管内部观

1.精原细胞；2.初级精母细胞；3.次级精母细胞；4.精子细胞；5.细胞分裂；6.间质细胞产生睾丸激素；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 Internal Sex Organs: The Production of Sperm

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.