. The cell in development and inheritance. Cells. GENERAL OUTLINE 235 of the four cells forms the " ovum " proper, while the other three, known as the polar bodies, are minute, rudimentary, and incapable of development (Figs. 89, 97, 114). In the male, on the other hand, all four of the cells become functional spermatozoa. This difference between the two sexes is probably due to the physiological division of labour between the germ-cells, the spermatozoa being motile and very small, while the ^gg contains a large amount of protoplasm and yolk, out of which the main mass of the embryo
. The cell in development and inheritance. Cells. GENERAL OUTLINE 235 of the four cells forms the " ovum " proper, while the other three, known as the polar bodies, are minute, rudimentary, and incapable of development (Figs. 89, 97, 114). In the male, on the other hand, all four of the cells become functional spermatozoa. This difference between the two sexes is probably due to the physiological division of labour between the germ-cells, the spermatozoa being motile and very small, while the ^gg contains a large amount of protoplasm and yolk, out of which the main mass of the embryonic body is formed. In the male, therefore, all of the four cells may become functional; in the female the functions of development have become restricted to but one Primordial ^erm-cell. Oogonia Primary oocyte or ovarian egg. Secondary oocytes (egg and first polar body). Mature egg and three polar " Division-period (the number of divi- sions is much greater). Growth-period. M aturation-period. Fig. 115. — Diagram showing the genesis of the egg. [After BOVERI.] of the four, while the others have become rudimentary (</. p. 124). The polar bodies are therefore not only rudimentary cells (Giard, '76), but may further be regarded as aboj-tive i\q-gs — a view first put forward by Mark in 1881, and ultimately adopted by nearly all investigators.^ The evidence is steadily accumulating that reduction is accomplished by two maturation-divisions throughout the animal kingdom, even in the unicellular forms; though in certain Infusoria an additional divi- sion occurs, while in some other Protozoa only one maturation-division has thus far been made out. Among plants, also, two maturation- 1 A beautiful confirination of this view is given by Francottes's ('97) observations on a turbellarian, Prosthecerceus. The tirst polar body is here often abnormally large, all grada- tions having been observed from the normal size up to cells nearly as large as the egg itself. Such po
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Keywords: ., bookcentury1900, bookdecade1910, booksubjectcells, bookyear1911
