The Sex Cells in Human Body

Eggs

The eggs (ova) of all the higher mammals are similar in both size and appearance round with a clear, thin, shell-like capsule as rigid as stiff jelly. The capsule encloses liquid in which hundreds of fat droplets, proteins, and other materials (including the nucleus) are suspended. The egg, the largest cell of the whole body, is approximately V200 of an inch in diameter-about one-fourth the size of the period punctuating the end of this sentence. The eggs of mice, rabbits, gorillas, dogs, pigs, whales, and humans are all about the same size.

Spermatozoa

The spermatozoa of different species can show greater differences in form than the eggs. The human spermatozoon consists of an oval head mostly occupied by the nucleus. With the aid of the additional magnification provided by the electron microscope, it has become evident that the head is covered by a saclike structure. This structure is called the acrosome and provides an outer membrane, the acrosomal membrane. The acrosome is attached by a short neck to a cylindrical middle piece that terminates in a thin tail about ten times as long as the head. The tail, which consists of several hairlike fibers resembling a horse’s tail, is capable of rapid side-to-side lashing movements. These movements propel the spermatozoon.

A spermatozoon can swim an inch in 4 to 16 minutes, depending on whether it is traversing watery uterine or tubal fluid, or relatively thicker cervical mucus. Easily blocked by the slightest obstruction because of its small size, its path is seldom a straight line, and it frequently takes more than the minimum time to progress an inch.

When ejaculated, spermatozoa are suspended in seminal plasma, a thick mucoid fluid produced by the male during sexual orgasm. Semen, which arises from the prostate, the seminal vesicles, and the other accessory reproductive glands, is a homogenous fluid immediately upon ejaculation. Within a few minutes it sets to a gel called coagulum. After about 15 to 20 minutes, this gel is fully re dissolved into a viscous fluid. In some cases this transformation is not complete, then the semen is very viscous and may have tapiocalike lumps.

A fresh drop of semen seen under a microscope suggests the rush of traffic in a crowded city street. Myriad spermatozoa dash here and there, now steering straight ahead, now halted by a speck of dust, now free again to scurry out of the microscopic field.