The Germ Cellis.--The great generalization on which modern embryology is based is the cell concept as applied to the gametes. This is the fact that the ovum and spermatozoon are single cells of the parent organisms, and cor related with this the relatively new knowledge of the physical basis of heredity as located m the chromosomes. It is essential to realize not only that the gametes are true cells, but that they are exactly equivalent as regards their chromatin content and consequently their heredity-carrying capacity (with the exception of the sex-chrosnosomes, for which see the articles CELL and HntEnrrv), and that their great diversity in size and form represents only a physiological differentiation by which the spermatozoon, mirmte and capable of locomotion is enabled to reach the egg, which as it is sup plied with foodstuff for the future embryo is much larger and non-motile. It is scarcely pos sible to conceive of two types of cells more widely different in form and appearance, yet both are the descendants of similar primordial germ cells, and their differences, except for the sex-chromosomes above mentioned, are entirely in the extranuclear s-tructures. The sperma tozoa are proliferated in the testis in enormous numbers. In their commonest form, often de scribed as tadpole-shaped, there is a head com posed of condensed nuclear chrotnatin, a middle piece containing a centrosome, and a vibratile flagellum or tail by means of which the spermatozoon is actively propelled and enabled to reach the egg. Frequently also a pointed body, the acrosome, is present at the anterior end and facilitates penetration into the ovum. There is no relation between size of sperma tozoon and size of organism. In man the entire length is 52-62 thousandths of a millimeter. In many minute invertebrates it is very much 'greater. In a few animals, the spermatozoa are non-motile and not of the usual flagellated form. The ovum, or egg, is always much larger than the spermatozoon, non-motile and usually of sphencal form. During the elaboration of the egg in the ovary granules of inert food-yolk or deutoplasm are stored up in its extranuclear protoplasm. This food-yolk is rich in protein, fats, lecithin, etc., and serves during develop ment as food for the embryo. The difference in size of eggs of different species is largely a difference in the amount of yolk and accord ing to distribution of this substance eggs are described as (a) alecithal or homolecithal, hav ing very little yolk evenly distributed as in the minute ova of mammals; (b) telolecithai, with the yolk massed toward one pole of the egg. the -condition Nertebrate eggs; and (c) centrolecithal, in which central mass of yolk is surrounded by a superficial layer of protoplasm, a type occurring in some arthro pods. The amount of yolk affects the develop ment of the egg profoundly. The largest eggs are those of sharks, reptiles and ,birds, which are of extreme telolecithal type and comprise the largest cells known. Those of placental inammals are very minute, that of man only 17 hundredths of a millimeter in diameter. In oviparous animals the eggs are usually en closed in protecting envelopes of which some are formed in the ovary and others secreted by the lining of the oviduct. In the hen's egg, to cite a familiar example, the delicate mem brane surrounding the yolk is of ovarian origin, while the albumen, shell membrane and shell are oviducal secretions. Frequently, as in in sects and bony fishes, the egg membrane is pierced by one or more minute pores, micro pyles, which permit ingress of the spermatozoon at fertilization.
Maturation.-- A phenomenon long known to be of almost universal occurrence ui the his tory of the egg is the successive extrusion from it coincident with or shortly preceding fertiliza tion, of two minute globules known as "polar bodies.° The significance of these bodies long remaVied a problem, the solution of which dur ing the later years of the 19th century consti tuted one of the most brilliant discoveries of cellular biology. It invested chromatin with a new importance, rendered possible a new un derstanding of germ-cells and fertilization and opened a new avenue for the investigation of the mechanism of heredity. It is a well-estab lished fact that the cells composing the body (somatic cells) of every animal contain a defi nite number of rods of chromatin called chrom osomes, this number characteristic of the par ticular species; also that these chromosomes are in even number and composed of two equiv alent groups derived respectively from the two parents (an exception to this occurs in the case of the sex chromosomes. See articles on
CELL and HEREDITY ). By a series of researches beginning in 1f383, in connection with which the names of E. Van Beneden, Theodor Boveri and Oscar Hertwig are especially identified, it was demonstrated that the ripe germ-cells of both sexes have only one-half the somatic number of chromosomes, though in the earlier primor dial germ-cells the full somatic number occurs. This reduction is accomplished through a phe nomenon lmown as "synapsis° or union in pairs of the chromosomes of paternal and maternal origin. Thus the somatic "diploid° number of singles chromosomes becomes reduced in germ cells to the °haploid° number of bivalent or double chromosomes, this reduction occurring in the spermatocyte or oOcyte cell generation prior to the last two cell divisions known as maturation divisions, by which the definitive gametic cells are formed. During the matura tion divisions the bivalent chromosomes are twice divided and the resultant univalent chromosomes distributed, still in haploid num. ber, to each of the four resulting cells. In the male these four cells all develop into func tional spermatozoa, but in. the female the di. visions are. so unequal as to consist merely in the successive extrusion from the egg of two abortive eggs or polar bodies. In some cases the first of these...bodies agah4 divides ,fio. that the end result is one functional egg and three polar bodies, which differ from the egg only in the smaller amount of cytoplasm and yolk, their chromatin content being exactly equiva lent. The racial significance of the reduction of the number of chromosomes to one-half in both gametes will be obvious in connection with the union of these cells in fertilization. Fig. 1.
"Fertilization" as applied to the union of gametic cells is a somewhat in adequate term, a relic of earlier days when it was supposed that the male semen merely acti vated the germ contained in the egg. While it is quite true that the spermatozoon does initiate development of the egg and thus "fertilizes" it in the same sense in which artificial treat ment with chemicals may fertilize many kinds of eggs, another essential fact of the conjuga tion of the two gametes. is the combination in the new zygote of two equivalent groups of chromosomes from the two parents. In many invertebrates and some aquatic vertebrates eggs and sperm are shed in the water, where conjugation occurs, but in many other animals the spermatozoa are transferred to the genital ducts of the female and fertilization is internal. Only one spermatozoon is normally concerned in the fertilization of an egg, though poly spermy, or the penetration of several into the egg-cytoplasm, frequently occurs, especially in forms having large eggs, but such supernumer ary sperms always degenerate eventually and take no part in the formation of the embryo. When a spermatozoon comes in contact with the ovum it penetrates the cytoplasm and in many cases a delicate membrane, the fertiliza tion membrane, is instantly secreted from the surface of the egg, thus preventing the entrance of any more sperms. At the same time other marked evidences of disturbance of the physico chemical equilibrium occur, often with violent streaming and new arrangement of formative zones in the protoplasm, and in some eggs the promorphology is rapidly established at this time. The tail, which is of no further use after the sperm has reached the egg, is frequently left outside. The head upon entrance speedily enlarges and assumes a vesicular appearance, becoming the male pronucleus. The egg nu cleus after the last maturation division is called the female pronucleus. Each of these pronu clei, as a result of previous reduction, has the haploid or halved number of chromosomes and by the union of pronuclei to form the zygote nucleus the normal diploid number characteris tic of the species is restored. Thus reduction maintains the specific number of chromosomes from generation to generation. A centrosome, the function of which is to initiate the process of cell-division, is also introduced by the sper matozoon, usually in the middle piece, replacing the egg centrosome which disintegrates after the last maturation division. The zygote, as the fertilized ovum is called, is now a complete cell, really a new individual in the stage of a unicellular embryo, with its chromatin, the ve hicle of heredity, derived equally from the two parents.