New experiments made on the eggs of sea urchins and other organisms have shown that conjugation may be replaced by an external irritating agent; for example, the action of certain chemical substances is sufficient to make eggs develop by parthenogenesis which would have died without this action. An entire being has been successfully produced from an egg divided into two by means of a hair. And even from the protoplasm of the egg without its nucleus, with the aid of a spermatozoid. We must not, however, base premature hypotheses on these facts. When a female cell, or egg, develops without fecundation (parthenogenesis) its nucleus enlarges and divides in the same manner as conjugated nuclei (mitosis). A point of general interest is what is called the _specific polyembryony_ of certain parasitic insects (hymenoptera of the genus _Encyrtus_). According to _Marchal_, their eggs grow and divide into a considerable number of secondary eggs, each of which gives rise to an embryo and later on a perfect insect. By shaking the eggs of certain marine animals they have been caused to divide into several eggs and thus to produce several embryos. All the individuals arising from the division of the same egg of _Encyrtus_ are of the same sex. [Illustration: PLATE I CELL DIVISION FIG. 1. Cell before division. FIG. 2. Division of centrosome. FIG. 3. Formation of chromosomes. FIG. 4. Dissolution of nucleus. FIG. 5. Lining up of chromosomes. FIG. 6. Division of chromosomes. FIG. 7. Division of chromosomes. FIG. 8. Attraction of chromosomes by centrosomes. FIG. 9. Concentration of nuclei. Division of cell. FIG. 10. Formation of new chromatin.] [Illustration: PLATE II FERTILIZATION OF THE OVUM BY THE SPERMATOZOID DIAGRAM OF OVUM AND SPERMATOZOID FIG. 11. _a_, Vitelline membrane; _b_, protoplasm, or vitellus; _c_, nucleus with chromatin; _d_, spermatozoid penetrating egg; _e_, another spermatozoid arrested by the vitelline membrane. FIG. 12. Formation of centrosome. FIG. 13. Formation of male nucleus by spermatozoid. Division of centrosome. FIG. 14. Development of nucleus of spermatozoid. FIG. 15. Nucleus of spermatozoid attains same size as that of ovum. FIG. 16. Formation of male and female chromosomes. FIG. 17. Lining up of male and female chromosomes.] =Embryology.=--It is not necessary to describe here in detail the different changes which the two conjugated cells pass through to become an adult man. This is the object of the science of embryology. We shall return to this in Chapter III. A few words are necessary, however, to explain the general principles.