Farm Animals - The Germ Cells
( Originally Published 1912 )
The spermatozoa were first observed in the product of the male organs in 1677, though their function was not then known. In 1827 the ovum was found and understood to be the seat of new life, though it was not until 1843 that the necessity of the union of ovum and spermatozoa was made clear and not until thirty years later that the significance of such union was realized. Though of unusual shape and make up, each of these reproductive bodies consists of but a single cell. A cell is a unit of structure in all plant or animal tissue as is a brick the unit in a wall. Growth consists of an increase in the number of cells, made possible by the material carried to the growing part by the blood. New cells produced by growth always resemble those existing in the part because they derive their principal and controlling part from the older ones. This controlling part or seat of the greatest activity is the nucleus which is shown at A in the ovum in Fig. 2b. Here the nucleus is small in proportion to the whole cell because of the extraordinary amount of outside material in the egg cell. The contents of the nucleus in a germ cell are believed to be the chief if not the sole vehicle of heredity between the offspring and the parent body within which the germ cell is produced. That the contents of these nucleii of all cells have some unusual qualities is evident from their behavior. It is the practice of the biologist to add clearness to the distinction of parts of material under examination by staining that material with chemical preparations. It invariably hap-pens that when living tissue is so stained the contents of the nucleus take on a deeper and more striking color than do other parts of the cell, evidencing a peculiarity of composition. For this reason the substance within the nucleus is called chromatin. Other grounds for attaching unusual significance to the chromatin are found in the intricate processes provided for in its division every time one cell becomes two cells. While the chromatin is dividing with striking exactness the outer part of a parent cell gives a half of itself to each new nucleus, but this halving evidences little or none of the design and exactness observed in the chromatin division. These facts apply with equal force to all body cells and to germ cells in their preparatory stages. The detail of the processes by which one cell becomes two cells is shown in Fig. 3. The unusual pro-visions for an equal and careful division of the contents of the nucleus, while the remainder of the cell divides with so little apparent system, lends color to the idea that the nuclear substance is of greatest importance to the resulting cells.
After division the chromatin again resolves itself into a granular condition and it is believed that substances pass out through the nuclear wall and control the entire cell and thus the direction of the development of the entire organism resides in the chromatin of its cells. It is sometimes claimed that the cytoplasm, the cell material outside the nucleus, exerts a controlling influence, but evidences of such may be due to presence of the chromatin that has migrated from the nucleus into the cytoplasm. In any body we may trace this chromatin material through successive divisions back to the original ovum and spermatozoon that originated the new being. This chromatin or hereditary material is present in all growing cells in the form of elongated and crudely cylindrical bodies spoken of as chromosomes. The number of chromosomes in the nucleii of the cells is the same throughout the body and never varies in the same species or class of animal. In our common animals the most accurate count possible shows the chromatin in each cell to be made up of sixteen chromosomes.
"The remarkable fact has now been established with high probability that every species of plant or animal has a fixed and characteristic number of chromosomes, which regularly recurs in the division of all its cells, and in all forms arising by sexual reproduction the number is even. Thus, in some of the sharks the number is thirty-six; in certain gasteropods it is thirty-two; in the mouse, the salamander, the trout, the lily, twenty-four; in the worm sagitta, eighteen; in the ox, guinea pig and in man the number is said to be sixteen."
It was said that the primitive or rudimentary germ cells multiply just as do cells in other parts, namely by each chromosome being split and donating half of its substance to the nucleus of the new cell. If the new animal produced by the union of a germ cell from either parent is to possess the number of chromosomes normal to a cell characteristic of its class some reduction of the number sixteen in the primitive bodies must be eftected, otherwise there would be a doubling up and hopeless confusion. The German zoologist, Weismann, in 1887, several years prior to the actual discovery, predicted that it would be found that the first form of the germ cells experienced some such reduction in the number of their chromosomes before reaching their mature form.
This process (maturation) of reducing the number of chromosomes in preparing a mature germ cell was first observed and understood about the year 1889. Since that time it has been seen to occur in sections from the ovaries and testicles of most of the larger animals and the process is a common subject of study in zoological laboratories. The special reducing process known in the female as o÷genesis; and as spermatogenesis, in the male, is apparently solely for taking from each germ cell one-half its chromosomes. No such thing occurs except with cells that are to be used in reproduction. In the male this process is continuous, and perfected spermatozoa are stored in considerable numbers. Maturation or reduction of the chromosomes of the female egg takes place quite rapidly and just prior to union with a spermatozoon. In some instances it is known to have occurred after the spermatozoon has passed through the wall of the ovum. Fig. 4 furnishes a parallel diagram illustrating the formation of spermatozoa and ova. This process of reduction is a basis for explaining many perplexing occurrences in breeding and is worthy of careful examination.
Fig. 5 shows the stages in the reduction of the chromosomes of an egg cell; only six of the chromosomes are shown. In farm animals each germ cell so reproduced would have eight chromosomes. The larger one with which the mass of food is retained is the mature egg or ovum; the other three perish. The processes of reduction of the number of chromosomes follow each other without intervals and are to all appearances solely designed to prevent the doubling of the number of chromosomes in the embryo which would follow if reduction did not take place. The later union of spermatozoon and ovum, each with one-half as many chromosomes as are normal to the species, restores the correct number in the fertilized egg from which the off-spring develops.
On the mother's side, then, the new animal is limited to receiving such qualities as were represented in the eight chromosomes that chanced to remain in the ovum. The sixteen originally present in the immature egg were derived in equal numbers from each grandparent. The process of preparing the male germ cells is altogether analogous to that observed in the female. There is no considerable accumulation of food within the male cell and the four bodies produced in the male organs are similar to each other in appearance and possibilities.
The spermatozoa, by virtue of the wriggling motion produced by their tail-like appendages shown in Fig. 2a. find their way to meet the ovum ordinarily within the tube connecting the ovary and womb. It is not known how long a time is occupied by the spermatozoa in reaching the ovum. The difficulty in procuring actual data is obvious. In one case in a rabbit, the ovum and spermatozoa were found united two and three-quarter hours after copulation. Though many spermatozoa attach themselves to the exterior of the ovum but one enters. Thus it is a matter of chance which eight of the chromosomes of the sire will meet the contribution of the dam. Considering the existence of the hereditary material in sixteen unit bodies and allowing for the variable tendencies contained in the individual chromosomes there need be no occasion for surprise when successive matings of the same parents fail to produce identical progeny. The spermatozoon having entered the ovum, their chromosomes coalesce and the seed or embryo of the new animal is complete. With favorable conditions growth and development ensue. The repeated divisions carry the leaven of the chromosomes to all parts of the body to form the most minute portions of the new individual. The development of the embryo is quite analogous to the production of a new plant from food secured from the natural sources and built up in the seed.
All of the non-nuclear material present in the fertilized egg is brought there by the ovum, but this material is not considered to convey influences of importance in the subsequent development.
As stock breeders, and therefore interested in the problem of heredity, we are not primarily concerned with the embryonic stages succeeding the union of the ovum and spermatozoon. Viewed in any way the production of a perfect foetus from the enlargements and divisions of two single special cells is a most marvelous process. Though marvelous it is no less comprehensible than is the development of a mature fruit-bearing plant from a single seed. The chromatin or the virtual seed material sends off its various component parts, and representation in the successive stages of change and the chromatin in any cell of the completed form is traceable directly back to the reproductive cells. The further study of this tangible vehicle of heredity is therefore of fundamental interest.