Page:The New International Encyclopædia 1st ed. v. 07.djvu/39

* EMBRYOLOGY. 25 EMBRYOLOGY. the female reproductive cell especially modified fur reproduction by being laden with a large amount of non-protoplasmic material, or yolk, intended as food for the embryo. The sper- matozoon is the male reproductive cell, specialized in the direction of rapid locomotion, since it has to move to reach and pass into the passive egg. The egg has evolved from a condition in which the two reproductive cells (or gametes) were alike (Paramecium, Spirogyra). In other species the gametes arc of different sizes, although both are motile. They are called macrospore and microspore. The former correspond to eggs and the latter to spermatozoa. In higher species, as in Volvox and the brown seaweeds, the macro- spore Incomes still larger and loses its motility. It has become a typical egg. Its increased size is due to the increased amount of food material stored in it. Development of the Egg in the Individual. Eggs, like spermatozoa, arise' from 'primordial germ-cells,' which are early differentiated from the body-cells and arc at first of no definite sex. Only later, frequently under the stimulus of food, do the germ-cells in certain individuals become male (spermatozoa), in others female (ova). In eoelenterates the germ-cells arise from either ectoderm or entoderm: in the higher groups from the lining of the body cavity. The primordial germ-cells that are destined to form eggs become intimately associated with the adja- cent tissue, from which they receive nutritive material and in consequence grow rapidly. At the same time the yolk-material, which is com- posed of spherules united by egg-plasm, is stored away in definite parts of the egg in the case of those eggs that have much yolk. During this period the nucleus gets very large and has re- ceived the name of germinative vesicle. The egg is a cell and as such has nucleus and cytoplasm. It is usually invested by one or more membranes, especially when it is discharged from the body to develop free. In respect to the amount and distribution of yolk, eggs may be classified as (1) alecithal, or with little yolk and that evenly distributed; (2) telolecithal. or with yolk concentrated at one pole, e.g. the hen's egg: (3) centrolecithal. or with yolk at the cen- tre, e.g. the eggs of most arthropods. See Egg. The number of eggs that ripen at one time is variable in different species of animals. In many mammals, as in man. there is usually one. In other mammals, two to ten. In birds the number varies from one to twenty. Land turtles may lay forty or fifty eggs. Many fish deposit several thousand eggs at one time. The egg is now ready for certain changes which precede fertilization. Reduction Division and Polar Globules. The process of fertilization would, if unre- strained, cause an enormous multiplication of chromosomes (see Cell) in descending genera- tions. This is averted by the fact that, previous to or coincident with fertilization, and after the egg has acquired its characteristic great size, one or more (usually two) parts are cast out or seg- mented off by a sort of modified cell-division from the eg^:. This casting out of the polar globules is the final act in the maturation or preparation of the egg for fertilization. It is an unequal cell- division, since the polar globules are many times smaller than the egg. The division by which the polar bodies is produced is known as reduction division. The first polar body may divide at the time the second is extruded, so that we have as a result of maturation three abortive eggs and the mature egg proper, which, when fertilized, develops into the embryo. Up to the time of the formation of the polar bodies the number of the chromosomes is the same as occurs in the somatic cells. As the cell gets ready for maturation, however, the number of chromosomes is reduced to half the usual number, and each of the remaining chromosomes is split in half at tin' formation of each polar globule. The polar bodies soon perish. In the development of the spermatozoon a sim- ilar reduction occurs, but in the process no cells are destroyed. The result of reduction is that the ripe germ-cells contain only one-half the number of chromosomes possessed by the cells of the body. This number is then restored in fer- tilization. The interpretation of reduction is ob- scure. As already suggested, it will permit of the union of new chromosomes without increase in amount. It has been suggested by Weismann that the process has this additional significance, that variation is produced in that certain of the parental qualities are eliminated from both sex- cells, so that the combination resulting from fer- tilization will be different for every individual. Fertilization of the Ovtjm. The eggs of most animals and plants require, before they be- gin their development, that a spermatozoon should penetrate into them. This act is known as fertilization. General Facts and Conditions. The ovum be- ing yolk-laden, or imbedded in nutritive material. is passive, and the spermatozoon has to come to it. In many of the lower aquatic plants and ani- mals the fertilization occurs in the water into which both kinds of sex-cells have been thrown. In the higher plants the ovum is deeply buried in the maternal tissues, and in most land animals it lies in the oviduct at the time of fertilization. In plants the pollen-grain sends down to the ovule a tube which conveys the spermatozoon, and in animals the spermatozoon reaches the egg in the oviduct by virtue of its locomotive capa- bility. The egg fertilized in the oviduct is then either extruded, 'laid,' to develop outside of the body, or it undergoes its embryonic development within the mother. Polyspermy. In the fertilization of most ani- mals normally only one sperm-cell enters each egg, but more may do so. The entraVice of the first sperm-cell causes the formation of a mem- brane, or the closure of the micropyle or perhaps affects the egg chemically, so that all other sperm-cells are excluded. In rare eases, however, several spermatozoa gain entrance to an egg. This is called polyspermy, and. it is said, results in an abnormal development of the egg. such as double cleavage, and soon ends in arrested development. In the eggs of certain insects and elasmobranch fishes polyspermy is frequent and normal, but in these cases only one of the sperm- cells normally unites with the egg nucleus. The supernumerary ones live for a time and may even divide., but their ultimate fate is obscure. Cytological Details. Fertilization consists essentially of mixing together living matter from two individuals — living matter which is there- fore dissimilar. This interpretation of the sig- nificance of fertilization is of rather recent date. The early embryologists thought the sperm gave