The Essential Reproductive Cells
Germplasm, the essential material concerned in reproduction, consists of sperm cells and eggs in sexual animals.
The detachable germinal units derived from the male individual are sperm cells. They are frequently called spermatozoa (sperma, seed; zoon, animal), an awkward and misleading word, embalming the historical fact that when these cells were first discovered by early microscopists, they were thought to be tiny independent parasitic animals.
A sperm cell in order to produce a new individual must always join forces with an egg cell. It can never become an individual unassisted. Such union, however, is not indispensable in the case of the egg cell, which among certain invertebrates may develop parthenogenetically, that is, without the assistance of a male sperm cell.
The sperm cells of different species have a chemical specificity for the eggs of their own kind, and animals do not bastardize under ordinary conditions even though their germ cells may have free access to each other, as in sea-water for example, where a variety of different kinds of eggs and sperm are present. If this were not true, untold confusion would result.
The motility which enables the active sperm to seek out the comparatively stationary egg is accomplished among vertebrates by the development of a vibratile “tail,” that sculls the cell body of the sperm forward through a liquid medium. Certain invertebrates, however, such as some of the nematode worms, arachnids, mites, myriapods, and many crabs, have amoeboid sperm cells which creep to their destination in contact with a substrate instead of swimming freely through an intervening fluid. Fish sperm do not take on an independent motility until they are expelled into the water.
The details of structure of a typical sperm, together with an indication of the sequence of events by which it attains a highly specialized locomotor form from its generalized embryonic shape, are shown in Figure 378. Cockerell has happily described a mature sperm cell as stripped “like a Roman soldier without impedimenta.” Its entire cytoplasm, indeed, is sacrificed to forming a structure devoted to carrying forward the “head” of the sperm, which is practically only the nucleus containing the chromosomal bearers of heredity.
The numbers of sperm cells produced by male animals of different species is greatly in excess of the number of eggs furnished by corresponding females. It has been estimated by Shipley that in man, for instance, the total number of sperm cells produced during the sexual life may be 340 billion, while the eggs that come to maturity during the lifetime of a human female will hardly exceed 400. This makes the ratio of possible sperm to eggs in humankind something like 850,000,000 to 1.
There is a corresponding discrepancy in size between the male and female germ cells, yet it is a demonstrated fact that the egg and sperm are essentially equal partners with respect to hereditary chromosomal determiners which each sex contributes to the mutual enterprise of a new individual.
The viability of sperm cells after detachment from the male likewise shows great variation. Haempel gives the duration of independent life in water of the sperm of certain fishes, as follows:
According to Lewis, human sperm may retain activity for three days after the death of the male, and if deposited in the female genital tract, for a week or more.
The sperm of some bats remain alive and efficient from the time of pairing in autumn until the following spring, when the eggs are ready for fertilization. Among invertebrates may be cited the remarkable case of the honey bee, where the sperm from the drone may live in the body of the queen for over a year.
The ova, or germplasmal cells of the female, are less independent of the individual which produces them than the sperm cells of the male. In many instances they tarry' within the protective body for a considerable time after attaining potential independence and may even undergo extensive development into a new organism, as for instance in mammals, before forsaking the maternal body in which they originated.
The fact that an egg is “fertilized” by sperm and not vice versa has entailed the necessity for providing various additions to the egg cell itself in the form of stored nutrition and protective envelopes for the forthcoming individual that are entirely unnecessary in sperm cells. The chief emergency ration stored in the egg is yolk, which represents one of the earliest instances of “preparedness” on record. The quantity of such food in the eggs of different species of animals varies all the way from a meager fat droplet in certain protoplasmic eggs to the relatively enormous supply making up the familiar yellow sphere in a hen’s egg. The large size of the latter is due to the generous supply of stored food material and not to the amount of living cytoplasm which is little different from that of much smaller eggs.
In addition to yolk, the eggs of turtles, crocodiles, and birds have a supply of albumen, or “white,” wrapped around the yolk that adds variety to the embryonic bill of fare.
The eggs of amphibians also are covered with a glairy albuminous coating, which has the property of swelling up into a thick protective jelly-like envelope upon exposure to water (Fig. 379). This explains why the total quantity of eggs which a submerged frog or toad lays all at once, enlarges and floats to the surface, forming a mass considerably greater than that of the entire body whence it came. The eggs of toads are strung together like pearls, while those of frogs and salamanders are in gelatinous clumps.
Eggs that are not shed directly into water, or do not undergo preliminary development into embryos within the sheltering body of the female, are provided with some sort of a protective shell. This may be leathery or of a texture like parchment, as in many reptiles, but it is usually calcified, being perforated by innumerable tiny air holes through which respiration takes place. The calcareous-shelled egg of the warm-blooded bird differs from that of the cold-blooded reptile by having: (1) a heavier firmer shell; (2) an air chamber at one end within the shell (Fig. 380); and (3) a greater amount of supplementary albumen, a part of which develops into the chalaza, that anchors the yolk at either pole like a twisted guy rope, preventing undue mechanical disturbance.
The chalaza also allows the yolk, bearing its precious protoplasmic disc, to rotate within the shell so that the disc is always on top and not pressed against the shell, regardless of the position of the egg as a whole. All of these additional modifications of the avian female germ cell are devices called forth by the necessity of egg-laying on land and subsequent incubation.
After an egg has taken in a sperm and is “fertilized,” it requires a period of enhanced temperature in order to begin development into an embryo. This is the period of incubation. Fishes usually resort to warmer waters to spawn and turtles deposit their eggs in sand where the heat of the sun has access to them. Sea turtles, whose young are liable to greater hazards than land turtles, come ashore and abandon on the friendly doorstep of sand and sun as many as 150 or 200 eggs, while alligators pile up a swampy nest of rotting vegetation in which to leave their eggs, the fermenting mass engendering the amount of added heat requisite for bringing the eggs to the hatching point. Frogs’ eggs, deposited in the shallow water along the margins of ponds in spring, receive the sun’s rays through their transparent spherical jelly-like envelopes which act like a lens in focusing the heat, thus providing the necessary increase in temperature. Birds build incubators in the form of nests where the temperature of their eggs is raised by means of contact with the warm bodies of the parents.
In shape the eggs of vertebrates are typically spherical, particularly those of fishes and amphibians that are deposited in water, as well as those of mammals which do not require a shell because they are not exposed. The eggs of reptiles are usually elongated and elliptical, while those of birds are prevailingly oval with one end more pointed than the other so that they pack economically within the confines of a nest. Most sea birds build shallow nests, depositing their eggs either in perilous crevices or in flat exposed situations, consequently their eggs taper so much that they do not roll away when disturbed, but simply pivot about in a circle, remaining safely in the nest.
As already pointed out, the egg by reason of the presence of yolk and albumen exceeds the sperm many times in size. The human egg is very small, but although only about % 25 of an inch in diameter, it is nevertheless 50,000 times larger in volume than a single sperm, which measures scarcely 5 micra in diameter.
The eggs of marine fishes are usually smaller and more numerous than those of fresh-water fishes, although elasmobranchs form a notable exception, as they have the largest eggs not only of any fishes, but also probably of any animal. Braus reports a specimen of the shark Hexanchus griseus that measured 4.2 meters in length and weighed 400 kilograms, from which he took out of a single oviduct 53 eggs of approximately the same size, each measuring 9 by 11 centimeters and weighing about 500 grams. The eggs of certain Japanese carcharid sharks are known to attain the size of 14 by 22 centimeters, dimensions considerably exceeding the average of 12 by 15 centimeters common to ostrich eggs.
The number of eggs produced bears a direct relation to the chances for attaining maturity. Elasmobranch fishes, the young of which are born alive and well advanced at birth towards a stage when they can fend for themselves, produce only a few eggs (Fig. 381). Prevost, for instance, gives four to fourteen ova as the seasonal output of the elasmobranch Torpedo marmorata. The stickleback, Gasterosteus, which makes a nest that is guarded by the male, lays less than a hundred eggs. On the other hand the codfish, Gadus, whose unprotected eggs are exposed to the countless perils of the open ocean, broadcasts several million eggs during a single breeding season. This enormous output in turn is numerically low when compared to that of certain termites among the insects whose queen lays eggs continuously at the rate of one per second for a year at a stretch, making a total of some 30 millions from a single female.
Ascending the vertebrate scale from fishes through amphibians, reptiles, and birds to mammals, there is an increasing provision for parental care with a corresponding falling off in the number of eggs produced that suggests the mathematical computations and conclusions of an expert life insurance actuary.
When eggs are supplied with a large amount of yolk, the embryos have the opportunity to reach a more advanced stage of development before hatching, but in the case of poorly provisioned eggs they must embark on their worldly adventures with less preparation. Mammals, whose eggs are practically devoid of stored nourishment, attain the advantages of advanced preliminary development by the device of gestation.