Asexual vs Sexual Reproduction

There are two fundamental methods that organisms use to reproduce:

• Asexual reproduction is reproduction from a single parent that creates a clone through mitosis
• Sexual reproduction is when two parents produce offspring with unique combinations of genes from both parents

Some multicellular animals use both asexual and sexual reproduction, while others only reproduce sexually

The sequence of events in sexual reproduction starts with gametogenesis, the formation of gametes

Gametes are specialized sex cells produced by each parent and there are two types: sperm and ova

Sperm, the male gametes, are created during spermatogenesis

Ova, the female gametes, are created during oogenesis

After gametogenesis, sperm and ova combine to form a zygote, a fertilized ovum

Fertilization can occur externally outside the bodies of both parents or internally inside the female

Early development of the offspring can also occur internally or externally

Some animals, like common garden snails, are hermaphrodites, organisms that have both male and female sexual organs

Hermaphrodites still reproduce sexually by cross-fertilizing each other's ova

After fertilization, the zygote must establish the three major body axes:

• The anteroposterior axis organizes the body from head to tail
• The dorsoventral axis organizes the body from back to belly
• The left-right axis organizes the body from left to right

Next, the zygote must undergo cleavage, the process by which the zygote rapidly divides without growing to become multicellular

Cleavage occurs early because a single cell is limited in the shapes it can form, and a single-celled zygote cannot begin differentiating into different tissue types

The formation of the three primary germ layers must be completed during early development, these three layers are: the ectoderm, the mesoderm and the endoderm

The ectoderm is the outermost layer of cells in the early embryo

The mesoderm is the middle layer of cells in the early embryo

The endoderm is the innermost layer of cells in the early embryo

Each primary germ layer will eventually become a specific set of tissue and cell types of the organism, so the formation of the different layers is one of the critical early events in animal development

After the three primary germ layers are formed, organogenesis, or the formation of organs, can begin in the embryo, and the different body structures can begin to take form