The Penis should be fully developed in a boy fourteen or fifteen years old. At that age the seminal fluid, destined for the perpetuation of the species, begins to be secreted by the Testicles, and not only excites the sexual organs, but affects every part of the body. The power of the mind, too, is improved; the digestion becomes more vigorous; the circulation of the blood is more rapid; indeed, every organ is abundantly nourished by it, and they perform their functions with much more energy than before this period of life. But the sexual organs, notwithstanding they may be of full size, have not yet matured, as will be shown hereafter. THE SEMEN. The Semen is a yellowish-white liquid substance secreted in the Testes of the Male, and which contains the animalculæ that grow to be future human beings. A microscopic examination shows it to consist of two substances, one fluid and the other little white globules. These globules soon dissolve when exposed to the air. The composition of the Semen is nine-tenths water—the remaining tenth being 10 soda, 30 phosphate of lime, and 60 animal mucilage containing a peculiar principle which is unknown. The living animalculæ of the Semen can be plainly seen with a microscope. They look like little eels with large heads, or pollywogs, and are supposed to be the germ of the brain and spine. They are called Zoospermes, and the microscope enables us to study their habits, and describe them with correctness. These animalculæ first grow in the little globules or eggs. Their extreme smallness may be imagined when we state that one of these globules, which are not visible to the naked eye, but can be seen only with a microscope, contains thirty of them. After the globules break open, the Zoospermes grow and become developed before they are in a fit state to enter the female Ovum. Attached to their head is a sort of sucker which fastens itself on to anything it can take hold of. By the aid of the microscope we are enabled to study the habits of these animalculæ so closely that it is known they cast their skins as they grow larger, like some insects. A French physiologist writes that he easily distinguishes their sex, male and female, but we doubt the truth of his assertion. They are apparently very playful and active, but it appears to be their nature to move only in one direction, and that straight forward—which movement has a connection with impregnation that is explained elsewhere in these pages. In watching the motions of these curious little creatures, we find that combats frequently occur among them. They fight very fiercely, and sometimes the fight lasts until only one or two out of a dozen or more are left alive, showing a pugnacity of disposition peculiar to mankind in the primitive or half-civilized state. They will live for several hours in warm water, and it is thus that their manœuvres are more easily seen. The Semen of a boy, before puberty,