Unqiue Journey Of New Sperm Cells
( Originally Published 1940 )
WE SHALL now note how, after maturation and separation, the sperm-cells find an exit along a very convoluted duct-system. Unlike that of the female organism this path, though uninterrupted, is very complicated. This is not surprising in view of the fact that an embryonic renal system has been utilised. We must descend into the mysterious workshop of nature, in the cellar of our body, where we find two explosives which constantly convulse the world. It is not a death-dealing explosion, however, but a life-giving one. Let us carefully examine the structure of such a testicle.
When it reaches its final shape just before birth, it is not spherical, but ovoid, bean-shaped like a small kidney, but rather less flattened. The small organ is surrounded by a very resistant capsule of connective tissue, and internally it contains no fluid but only solid material. On dissection it is seen to be made of count-less finely convoluted tubules which have been referred to and which under the microscope look like ordinary gland-ducts. Internally they are lined by a layer of cells which look like gland-cells but which are derived from the epithelial cells which were segregated in the embryo. To the naked eye these canals appear as thin threads, like a torn piece of crochet-work, all tightly packed into longish cones, which are separated from each other by firm connective-tissue septa. These septa are really prolongations of the external capsule which project between the cones and become thinner as they proceed. The whole scheme is really somewhat similar to that of the kidney. The softer tissue between the fine canals is the interstitial tissue which in all probability manufactures as its metabolic product, the above-mentioned typical organochemical substances governing sexual differentiation. The apices of these cones are directed towards a single point at the centre of the slightly concave side of the testicle corresponding to the point in the kidney where the efferent duct and the blood-vessels emerge, and to the spot in the bean where the embryo is situated.
These fine canals gradually amalgamate to form larger ones (thus giving the cones their shape) and at the point where the cone-tips meet, and larger canals emerge, these larger canals, still strongly convoluted, form a thickening, the epididymus, at the hilum of the testis. In this way, the reniform shape is disguised, and the whole organ appears somewhat ovoid like the phalanx of a finger.
In the epididymis, the larger canals converge further to form a single duct which finally emerges at one pole of the testicle, sus-pending the latter lengthwise.
This duct which is firm-walled and unconvoluted is known as the vas deferens. As mentioned, it may be felt as a thickened cord through the skin of the scrotum, and seems as if it were the stalk by which the testicle remains suspended after its descent. In reality, it is the first part of the male genital canal. The sperm-duct runs upwards from the testicle along the groin, enters the abdominal cavity, describes a graceful curve at the side of the urinary bladder, and proceeds to the posterior aspect of its base. Here the two sperm-ducts converge and open with very fine apertures into the wide urethra. From this point onwards the urethra serves to convey both urine and semen.
Just before opening into the urethra each sperm-duet is provided with a lateral reservoir, the seminal vesicle. These are elongated and lie on each side against the posterior wall of the bladder.
When we use the terms seminal canal and seminal vesicle, we must remember that these are traditional names bound up with traditional errors. If one chooses names drawn from plant life, one would not say semen (seed) but rather pollen, for seed is the end result in the ripe fruit. In some races, however, and especially among the ancients, we find the view that the male supplies the seed and the female is simply the field in which it is sown. This is surely the height of male vanity. (Vide chapter 50.)
This whole apparatus remains in its rudimentary latent condition till the age of puberty. Even the cells lining the minute canals remain inactive until puberty, since they are not glandular cells. We have shown in the previous chapter how, after puberty, sperm-cells are formed in millions by a new and unusual type of cell division. They are detached with a minimum of moisture and are simply pushed along by the newly formed cells behind them. In their further course in the epididymus, they are transported more actively by the ciliated epithelium which lines the tubes there.
Finally, the sperm-cells reach the two lateral appendages: the seminal vesicles. Many enter these and mix with the mucous secretion of their walls while others collect in the seminal-duct and only mix with the mucous secretion, when, at the right time, a strong contraction of the seminal vesicles and the seminal-duct occurs. The resulting mixture of mucus and sperm-cells is known as the semen, and is ejected into the urethra and thence out of the body.
Up till now the sperm-cells could only remain motionless like the egg-cells, but now there is enough space for them to move actively. A sperm-cell under the high power of the microscope appears like a Iongish dot (the part of the mother-cell east off at the reduction division), provided with a curved hair-like tail (flagellum or cilium). The whole consists of almost fluid protoplasm. Have you seen frog-spawn in an early larval stage in an aquarium, when the black spot acquires a filiform tail and rushes about in the water? This bears a crude and highly magnified, but pretty accurate, re-semblance to the movement of the sperm-cells, when a sufficient quantity of fluid medium enables them to become active.
Like a boat propelled by the spiral movements of a single oar at the stern, the small spot, the sperm-cell, is propelled forwards by the wavy motions of the flagellum at its hinder end. Every freely moving cell is attracted by food and other useful chemical substances (positive chemotaxis) and repelled by poisons and other noxious influences which would coagulate protoplasm (negative chemotaxis), And so, as soon as the sperm-cell, which is almost devoid of deuteroplasm (yolk), enters the female genital organ, it is attracted by positive chemotaxis towards the egg-cell, which is of far greater size and contains much deuteroplasm.
A sperm-cell consists of little more than a nucleus and a flagellum, while an ovum, like any ordinary cell, has a large amount of deuteroplasm surrounding the nucleus. Under exceptional cumstances, in some lower animals, it is possible for an egg-cell to live and develop into an adult individual without conjugation with a sperm-cell (Parthenogenesis), but we know of no instance of a similar development of a sperm-cell without the aid of an egg-cell.
The sperm-cell and egg-cell differ greatly in size. The latter, which is spherical, has a diameter of 17 millimetres and can just be seen with the naked eye, while the former, including the tail, is only .05 mm. long. The passage of the sperm-cell along the moist walls of the vagina and uterus is like a great ocean voyage. For these delicate creatures it is a fateful journey fraught with the danger of utter destruction. Of the 200 million sperm-cells discharged at a single ejaculation, only one survives if fertilisation ensues.
The total number of egg-cells present in the ovary of a woman is far smaller. Heide found in the ovary of a girl aged 18 years 36,000 egg-rudiments; Sappey in a girl aged 3 years found 400 thousand egg-rudiments, of which three or four hundred probably reach maturity and are cast off.
Even in the case of twins, at most only two egg-cells and two sperm-cells may survive. All the rest are destined by nature to perish in the maelstrom. This fecundity is Nature's device to ensure the continuance of the species.