A first book in organic evolution

by D. Kerfoot Shute

It will thus be observed that the centrosomes and their achromatin fibers are a beautiful mechanism by which the heredity threads (chromosomes) are exactly divided into two equivalent halves.

There are some cells (Amœba proteus, for instance,) which divide in a much simpler manner than by mitosis; in these there is no complicated rearrangement of the chromosomes and no disappearance of the nuclear membrane, the nucleus simply becoming separated into two parts (Amitosis).

The Human Ovum. The human ovum is a typical cell about one-fifth of a millimeter in size and spherical in shape. It is a nucleated piece of protoplasm possessing an enveloping cell-wall (metaplasm). The protoplasm contains nutrient material or yolk (metaplasm). The maturation of the ovum essentially consists in throwing out half of its chromosomes. In doing this the nucleus (mother) approaches the surface of the protoplasm ([Fig. 9], A), and divides, by mitosis, into two daughter nuclei; then the unripe ovum divides into two cells, but of very unequal size. This process is repeated a second time. Thus two small cells are formed which are known as the polar bodies ([Fig. 9], A, B, pol. b). The large cell remaining after the formation of the two polar bodies is the mature ovum (B). Its nucleus, which recedes towards the center of the protoplasm, is called the female pronucleus ([Fig. 9], B, f. pr.). The pronuclei of mature ova differ from the nuclei of all the other cells of the body in that they only contain half as many chromosomes (hereditary threads).

The ripe spermatozoid (a flagellate sexual cell of the male) corresponds to the ovum of the female. It also contains a pronucleus having only half the number of chromosomes that the other cells of the adult body possess.

Fertilization. The male sexual cell (spermatozoid) is vastly smaller than the female sexual cell (ovum). Having a flagellum ([Fig. 9], B, s), it moves about, like a tadpole in water, and seeks the ovum. When it comes in contact with the ovum it penetrates into its interior (usually only one doing so), as indicated at ([Fig. 9], B, s). The tail or flagellum of the spermatozoid fuses with the protoplasm of the ovum, and disappears from view. Its pronucleus (C, m. pr.), accompanied by its centrosome (C, m. c.), approaches the female pronucleus (f. pr.) of the ovum ([Fig. 9], C, D). Finally the male and female pronuclei coalesce to form a single nucleus ([Fig. 9], E, n. os.). The centrosome of the ovum persists for awhile and then disappears; that of the spermatozoid remains in the ovum, and is the agency by which cell multiplication, through mitosis, takes place.

Fig. 9.—Diagram illustrating the maturation and fertilization of the human ovum. A, one polar body is formed and a second is in process of formation; B, both polar bodies are formed and a spermatozoid is penetrating the ovum; C and D represent the approach of the male pronucleus towards the female pronucleus; E indicates the amalgamation of the two pronuclei to form the nucleus of the oösperm (segmentation nucleus); pol. b, polar bodies; pol. c, centrosome of the polar body; chr. p, chromatin of the polar body; f pr, female pronucleus; p, protoplasm; p p, peripheral protoplasm (but not cell wall); f c, female centrosome; m c, male centrosome; m pr, male pronucleus; n, os, nucleus of the oösperm (first stage of a human being).

The cell resulting from the coalescence of the male pronucleus with the pronucleus of the ovum, and which is only one fifth of a millimeter in size, is the first stage in the existence of a human being. Man thus starts his career as a Protozoan-like creature,—as a unicellular animal. The fertilized ovum is called the sperm-egg (oösperm), and contains now the normal number of hereditary threads (chromosomes); for those of the male have been added to those of the female.

Segmentation of the Oösperm. The fertilization of the ovum imparts to it a wonderful stimulus, so that the oösperm divides, by mitosis, into two cells, these two into four, the four into eight, the eight into sixteen, these into thirty-two, and so on repeatedly, until a large number of comparatively small cells are formed ([Fig. 10]). This mass of cells is spherical in shape, and the little round cells towards the surface project in such a way as to give to the mass an appearance somewhat similar to the fruit of the mulberry, whence it is termed the mulberry body or morula ([Fig. 10], 4). In the morula stage of his existence man resembles the solid colony of protozoans represented by Pandorina. The cells of the morula next become arranged regularly in a single layer at the circumference, by which the embryo assumes the form of a hollow sphere, and is known as the blastula ([Fig. 10], 5). This phase of man’s existence is quite suggestive of Volvox.