What is Sexual Reproduction

The development of a new organisms in sexual reproduction begins with two cells, one being a sperm cell and the other an egg cell.  The egg cell provides the nutrients in the form of embryonic fluid for the organism, while the sperm cell is specifically designed as a transporter of DNA to ensure genetic diversity. 

Initial Contact and the Fast Block

When the sperm contacts the egg, the sperm is drawn to the egg via a chemical excretion by the egg itself.  If the sperm and the egg are the same species, a tube is created by the head of the sperm through the membrane of the egg allowing it to enter.  At the point of contact sodium channels are opened allowing an influx of positively charged sodium channels, the rapid change in voltage on the inside and concurrent negative charge on the outside membrane of the egg blocks other incoming sperm from binding.  This stage is termed “The fast block to Polyspermy.” 

The Next Stage and Slow Block

The next stage which takes longer and is correctly labeled “The Slow Block to Polyspermy”.  As the sperm and egg cell fuse, the egg cell’s endoplasmic reticulum releases stored calcium which when bound to the plasma membrane permanently ensure no other sperm enter the cell. 

Formation, Division, and the Four Stages

The sperm and the egg cell separately only have half the set of necessary chromosomes and are termed haploid.  When the sperm and egg cell nuclei combine, each half set becomes on full set which is genetically different from either parent.    The cell is now a zygote.  The single zygote cell then duplicates multiple times replicating exact copies of its DNA in a process called mitosis.  It is important to note that the cytoplasm volume is shared by all cells established from the original cell to ensure the correct size of the forming embryo and its necessary nutrients for success. While the cytoplasm is shared, the chromosomes themselves duplicate before cell division generating to identical copies of DNA before division.  The division of these cells is a four stage process.  The first is Prophase in which the chromosomes of the cell condense into tight bundles.  Next is Metaphase in which the bundles line up in the center of the cell.  In Anaphase each chromosome pair is pulled to opposite sides by spindle fibers aligned on their designated side.  Finally, Telophase is when the membrane of the cell splits forming two cells from its original. 

Direct Development

Two major types of sexual reproduction are used by most organisms.  One is the familiar to mankind, involving a close relationship between two organisms and the deliberate introduction of the sperm and egg cell.  This type of reproduction generally produces few fertile eggs and requires the parents to invest a great deal into the care of these eggs to ensure their survival. In this type of reproduction, the parents are heavily invested in their offspring.  The development of the new generation is almost entirely dependent upon the protection of their parents.  This is called direct development.  When the offspring hatch from the egg, they are fully formed and even in the state of infancy resemble their parents. 

Indirect Development

The other form of reproduction is called mass spawning in which many sperm and many eggs are released freely in hopes that they unite and that the offspring survive by simple probability due to the number available.  The parents in this indirect development form do not invest in the eggs and the species in general relies on chance and statistical probability of success due to vast numbers.  These eggs are often forced to hatch earlier than eggs involved in direct development and the offspring commonly hatch in an intermediate stage in the form of larvae. The designation for when an egg needs to hatch is dependent on the amount of nutrients they have available to them which will be discussed later.  

The Importance of Medium

Both forms of reproduction have proven successful but only under certain environmental circumstances.  If there is not a sufficient medium such as water to supply transportation for sperm and eggs, than mass spawning is most likely not exercised and general courtship is used.  If however there is a sufficient medium and possibility a lack of mobility of the parents directly to create a direct introduction of sperm and eggs, than it is likely mass spawning is the choice for the species. And the organisms live in set groups in close proximity to one another to better the chances for propagation. 

Amount of Yolk and Development of Characteristics

The yolk of the egg provides the necessary nutrients to sustain the embryo as it develops.  The amount of this yolk varies drastically from species to species. The more yolk that is contributed to each egg is directly correlated to the amount of parental care required and inversely correlated to the amount of eggs actually produced.   Isolecithal eggs have little yolk and it is dispersed throughout the cytoplasm, it is the egg classification with the least amount of yolk.  This type of egg would occur during mass spawning and indirect development.  And requires little parental care to none at all.  Mesolecithal eggs have more yolk that usually makes up one half of the cell.  A parental contribution is required to ensure these eggs are successful.   Telolecithal eggs have maximum yolk which are almost entirely full of yolk and require a heavy parental investment and would most likely show where few eggs are created and direct courtship is the form of reproduction.  In Isolecithal eggs, multiple divisions take place because there is room to create these divisions due to lack of yolk and abundance of free space.  In telolecithal eggs less division is possible initially because the amount of yolk is creating a hindrance to many divisions. 

Animal zygotes create a blastula.  There are many forms of blastula, but blastulas in general are a defining characteristic of Kingdom Animalia. Blastula consists of an ectoderm on the outside, an inner endoderm and in most cases a middle area called a mesoderm.   The significance of forming a blastula is that it allows the process of gastrulation.  Gastrulation is the formation of a gut or formal digestive area.  The endoderm is formed by the invagination of the outside of the blastula which forms the digestive tract.  The outer ectoderm forms the nervous system and the mesoderm will form circulatory and muscle system, but only if present. 

And thus an embryo is is in development, a bun is in the oven, something is cooking.