Germ cells usually consist of an egg and a spermatozoa.
Eggs and spermatozoa differ from each other both morphologically and physiologically.
The Egg or Ovum
Eggs are usually spherical or oval in shape, although they may vary greatly from the typical form and may even be ameboid as in certain ccelenterates.
In size they range from that of the mouse, which is only about 0.065 mm. In diameter, to that of birds, which are several inches long. The large volume of the latter is due to the presence of an enormous amount of nutritive material, and the general statement may be made that the size of an egg does not depend so much upon the size of the animal as upon the amount of yolk stored within it. The egg nucleus, which is frequently very large and clear, is known as the germinal vesicle; and its nucleolus has often been referred to as the germinal spot. Embedded within the cytoplasm of the ovum are several bodies besides the yolk globules.
A "yolk nucleus" may be present; mitochondrial granules or rods may occur ; and special inclusions, which become associated with the primordial germ cells and have been named keimbahn-determinants, have been recorded in many cases.
Considerable evidence has accumulated that the egg substance is not a homogeneous, isotropic mixture, but is definitely organized, and that this organization is related to the morphology of the embryo which is to develop from it ; hence we speak of the promorphology of the egg. Eggs are said to possess polarity, and even the oogonium as it lies in the ovary is definitely oriented with respect to its chief axes.
The principal poles are dissimilar ; the end of the egg containing most of the cytoplasm and nearer which lie the nucleus and centrosome is known as the animal pole ; the other end, which is often crowded
Figure Germ cells. Ovarian ovum of a cat just before maturity.
C. m. = cell membrane; mics. = microsomes; ncl = nucleolus; n. m = nuclear membrane; yk, al. = yolk alveoli. (From Dahlgren and Kepner.)
With the yolk globules, is called the vegetative pole. The subject of the organization of the egg will be referred to more in detail later.
The male sex cells or spermatozoa differ very strikingly from the eggs. They are usually of the
flagellate type, consisting of a head, largely made up of chromatin, a middle piece, and a vibratile tail. Spermatozoa are comparatively minute, ranging in size from those of Amphioxus, which are less than 0.02 mm. Long, to those of the amphibian, Discoglossus, which reach a length of 2.0 mm.
According to Wilson it would take from 400,000 to 500,000 sea urchin spermatozoa to equal in volume the egg of the same species. It is not surprising, therefore, to find that the number of spermatozoa produced by a single male may be hundreds of thousands times as great as the number of eggs developed in a female.
Eggs are, as a rule, incapable of locomotion, but spermatozoa are active, swim ming about by means of their tails until they reach the passive eggs which they are to fertilize.
Since generally only one sperm-flagellate spermatozoon fuses with an egg, it is obvious that most of them never perform the function for which they are specialized ; but apparently an enormous number are formed to make the fertilization of the eggs more certain.
The experiments of Loeb and Bancroft (1912) on spermatozoa showed that when the living spermatozoa of the fowl are placed in a hanging drop of white of egg or in yolk they undergo a transformation into nuclei. The possibility that a spermatozoon may give rise to an embryo without the help of an egg is recognized, but this has not yet been accomplished.
