If you've read the section regarding sexual behaviour, then you read a little bit about the reproduction of penguins. In this section, I'll give it serious development.

    Penguins reproduce by sexual means. "Sexual reproduction" implies that two gametes -- single cells -- unite to form one single cell or zygote. The female gamete is the ovum and the male gamete is the sperm. After copulation occurs and assuming sperm make it to the magnum of the female reproductive tract, the sperm penetrates the ova. This is called fertilization. The resulting, unified cell is called the zygote. Look at the drawing to the right. First we'll trace development and the path of the sperm. Sperm develop in the testes; penguin males have two of them closely apposed cranioventral to the first kidney lobe in the abdomen. (Remember the terms cranial and ventral?) The sperm then travels into the ductus deferens which runs in medially and enters the caudal cloaca (bottom of the cloaca) in a specific region known as the urodeum. During copulation, cloacal contents are delivered to the female cloaca. If the copulation is timed with ovulation, fertilization may occur.

    Females usually have only one ovary; usually the left ovary persists. A developing follicle supports a developing oocyte (immature ovum). As the oocyte develops, fluid built up into the follicle raises intrafollicular pressure to the point of rupture. The oocyte is expelled into the body cavity.

    Ovulation occurs, releasing an ovum into the body cavity (technically this is called the peritoneal cavity, but we won't go into that distinction). The ovum is swept into the infundibulum by the fimbria of the infundibulum. Fimbria are finger-like projections of the infundibulum that become engorged with blood during ovulation. The maturing ovum moves into the magnum where it encounters the sperm. Fertilization occurs and the zygote, or "egg," continues to travel down the female reproductive tract. As the egg continues its travels, it acquires a layer of albumen and the shell membrane begins to deposit in the isthmus of the tract. All the while, the size of the yolk sac and egg is increasing. As the egg passes through the uterovaginal junction, it hardens extensively and is passed into the world through the cloaca.

    The picture to the left is of an early embryo. Quick inspection will show you that the embryo at this stage is overwhelmed by the extensive yolk sac which in this case imparts a yellow color to the entire contents of the egg. It is from the yolk sac that the developing penguin embryo draws all its energy. There's plenty of necessary fats, cholesterol and sugars in this area of the egg. Developing neurological systems require fats and cholesterol, while most energy is derived from the metabolism of simple sugars like glucose, galactose, and fructose.

    You can see an extensive network of blood vessels stemming from the embryo to the recesses of the yolk sac. Nutrients enter the blood vessels and they traverse to the embryo proper. At this point, nutrients can be distributed throughout the developing embryo.

    Within a few days, the embryo grows in size to resemble something like a animal form. The embryo to the right has been incubating for several days. At this point, it is still drawing nutrients from the yolk sac. Its oxygen is supplied from the external world; it enters the egg through microscopic pores in the shell. Metabolic carbon dioxide exits the shell by these same channel pores.

    The picture to the right shows the developing embryo. The most prominent feature of this embryo is the developing eye. Close inspection will reveal unrotated limb buds (during development, upper limb buds will rotate laterally and lower limb buds will rotate medially). Vascularization (blood vessel development/presence/etc.) is extensive during this stage. The primary artery delivering nutrients to the embryo can be seen in the median plane of the embryo, entering the ventral surface. The dorsal surface of the embryo is unremarkable in this picture, but the vertebral column is developing.

    A slightly earlier staged embryo. Several features are prominent in this sagittal section. Moving cranially to caudally, we can see the head of the penguin developing its eye and brain (forebrain). At this stage, the heart is prominent as a ventral diverticulum of the ventral surface. At about this level on the dorsal side of the penguin embryo is the developing auditory center. It is indicated here as the otic vesicle.

    Running through the dorsal surface of the penguin embryo from cranial to caudal regions is the neural tube. The neural tube develops from the ectoderm of the embryo. The ectoderm is the outermost cell layer of the embryo. The neural tube formation is induced by the notochord, a structure shared by all Chordate animals. (Not all Chordates develop vertebrae, however. That is why birds are listed with the sub-phylum Vertebrata to note that they are Chordates with vertebrae.) The neural tube will develop into the central nervous system which includes the spinal cord and the brain. The notochord will regress into the intervertebral discs.

    The last feature of this embryo section are the somites. Somite function can be difficult to understand. They are condensations of cells around the neural tube that form the vertebral system. They unite with the notochord and surround the developing spinal cord. Once they coalesce around the spinal cord, they fuse together to form the vertebrae and the base of the skull. The development of the vertebral column and base of skull is actually more complicated than that, as they have to permit exit of nerves from the spinal cord throughout the length of the spinal cord, so the somites don't completely fuse around the spinal cord.

    Incubation period varies among penguin species, but the chick eventually hatches from the egg if the conditions are right. I won't go into chick development, but know that there is a gradual process of growing and plumage changes before the chick becomes a juvenile and before the juvenile becomes an adult. Each chick goes through the following stages: natal down to juvenal plumage via the prejuvenal molt. From the juvenal plumage, the penguin moves to its basic plumage via a prebasic molt. Now we'll go on to penguin systems.