This kidney system is analogous to the kidneys of the primitive fishes. However, in human embryos the pronephros in transitory and nonfunctional. The bilateral pronephros appears in the beginning of the 4th week as 7 to 10 clusters of mesodermal cells located in the cervical part of the intermediate mesoderm. These cell clusters form rudimentary excretory units called nephrotomes. The nephrotome consists of glomeruli and their associated pronephric tubules. The pronephric tubules terminate in an uncanalized pronephric duct. Being transitory and nonfunctional, the pronephric system disappears completely by the end of the 4th week of development. Some embryologists are of the view that in human embryos the pronephros is not formed at all.