Reproduction existed long before sex; asexual reproduction may have predated sexual reproduction by 1.5 billion years or more.  Although sexual reproduction does have the advantage of creating diversity in a species, it is certainly not a requirement for life in general.  Asexual reproduction is the predominant form of reproduction in bacteria and in many protists and fungi.  It can occur in plants and animals as well, including vertebrates.  Although sexual reproduction is the primary form of reproduction in animals, the process of meiosis which is the basis for sex was essentially complete before the first animals evolved. Although animals would make sexual reproduction more complex (by enclosing meiotic cells together to form gonads, developing tubes through which gametes could travel, adapting the process to occur inside the female body in many groups, and recruiting the nervous system to involve sexual drives and courtship behaviors) the cellular mechanisms of sexual reproduction have changed little since the evolution of eukaryotic protists.

     In sponges, sexual reproduction occurs without gonads—sex cells may originate from anywhere any part of the animal (Beklemishev, vol. 2).  In sponges, ova move like amoeba and may perform phagocytosis (Barrington, p. 386).  Spermiogenesis occurs to modify the shape of the male gamete, although no acrosome is formed on the sperm (Harrison, Vol. 2).  Oocytes are large cells, about ten times the size of the oogonia from which they developed (Harrison, Vol. 2). 

HYDRA

     The most primitive flatworms are equivalent to coelenterates with regards to their reproduction. Acoela possess no gonads since spermatogonia and oogonia are dispersed throughout the body.  Eggs lack yolk (Hickman, Beklemishev, vol. 2).  Higher flatworms do possess gonads and in nemertine worms, the gonads are lined by epithelium (Hickman).  In lancelets, the gonads are segmented unlike the situation in higher chordates.  Lancets have 26 pairs of gonads, starting in the middle of the pharynx. 

     In jawless and cartilaginous fish, the secondary oocytes are released into the body cavity.  Even though females in higher vertebrate groups possess an oviduct to transport the secondary oocytes, they are still released into the body cavity first (causing a risk of ectopic pregnancy in humans) (Weichert, p. 280)

     Many invertebrates are hermaphrodites, possessing both male and female gonads.  This condition even exists in primitive chordates: almost all ascidians and some lancelets are hermaphrodites.  In lampreys, 2 larval gonads fuse to form 1 which is attached to a mesovarium.  In hagfish, the anterior portion of the gonad becomes ovarian tissue and the posterior portion becomes testicular, although these do not function simultaneously. In most fish, the two ovaries fuse.  In cartilaginous fish, the left ovary degenerates later in life (Weichert).

BOWFIN

FROG

FOLLICLES

The simplest animals, sponges and cnidarians, lack follicular cells which surround the ova. In some flatworms the ovary is compacted and divided into layers so that oocytes may be surrounded by follicular cells.  In hemichordates and urochordates, follicle cells surround oocytes unlike the condition in males which possess no such cells.   In both hemichordates and urochordates, oocytes are released from the ovary before meiosis is complete.  Just as in vertebrates such as humans, an ova is technically not produced until fertilization (and thus a female which has not conceived has technically never produced a female gamete). (Benito, form Harrison 1997, p. 93; Burighel, from Harrison, 1997, p. 282).  In the most primitive flatworms, eggs lack yolk (Hickman, Beklemishev, vol. 2) while in others, entolecithal egg production occurs (Dougherty). 

     In tunicates, some of the follicular cells remain as a “corpus luteum” or follicular residuum after ovulation. (Burighel, from Harrison, 1997, p. 288)  In a number of teleost fish, a corpus luteum forms after ovulation (Weichert, p. 281).  In amphibians, the ova are released from the external portion of the ovary and the center portion functions in the lymphatic system.  Corpus lutea exist in some amphibians, all reptiles, and all mammals (Weichert, p. 283; Romer).  In the Indian elephant, blue antelope, and in horses, accessory corpora lutea form during pregnancy (Weichert, p. 286).

AMPHIOXUS

OOCYTES

SALAMANDER

CHICKEN

OPOSSUM

CAT

PIG

MONKEY

HUMAN MODEL

    Marsupials are similar to placental mammals in their estrus cycle which includes the cyclical growth and loss of the endometrium (Stonehouse, 1977).  In a number of mammals, ovulation only occurs after copulation. (Weichert, p. 285)

SPERM

HYDRA

     In hemichordates and cephalochordates, the sperm consist of a head, a midpiece containing few mitochondria, and a flagellar tail just as in chordates (Benito, form Harrison 1997, p. 95 ).  In urochordates and lancelets, intercellular bridges exist between spermatocytes during spermatogenesis, just as in vertebrates (Burighel, from Harrison, 1997, p. 295)  In lancelets, spermatocytes progress towards lumen as meiosis occurs and Sertoli cells exist (Ruppert, from Harrison, 1997, p. 491).  In both lancelets and jawless fish, sperm are released into the coelom (as are ova).

Amphioxus

SHARK

LUNGFISH

FROG

TURTLE

ALLIGATOR

CAT

MONKEY

HUMAN MODEL