Animal Development- Part I

The study of development of embryo is referred as Embryology. The history of Embryology dates back to  18th century, the prevailing notion in human embryology was preformation: the idea that" semen contains an embryo — a preformed, miniature infant, or "homunculus" — that simply becomes larger during development. The competing explanation of embryonic development was epigenesis, originally proposed 2,000 years earlier by Aristotle. According to epigenesis, the form of an animal emerges gradually from a relatively formless egg. As microscopy improved during the 19th century, biologists could see that embryos took shape in a series of progressive steps, and epigenesis displaced preformation as the favored explanation among embryologist.
Modern embryological pioneers include Gavin de Beer, Charles Darwin, Ernst Haeckel, J.B.S. Haldane, and Joseph Needham, while much early embryology came from the work of Aristotle and the great Italian anatomists: Aldrovandi, Aranzio, Leonardo da Vinci, Marcello Malpighi, Gabriele Falloppia, Girolamo Cardano, Emilio Parisano, Fortunio Liceti, Stefano Lorenzini, Spallanzani, Enrico Sertoli, Mauro Rusconi, etc.^]
After the 1950s, with the DNA helical structure being unravelled and the increasing knowledge in the field of molecular biology, developmental biology emerged as a field of study which attempts to correlate the genes with morphological change, and so tries to determine which genes are responsible for each morphological change that takes place in an embryo, and how these genes are regulate.
There are 5 stages of animal development:

1)      Gametogeneis

2)      Fertilization

3)      Clevage

4)      Gastrulation

5)      Organogenesis

In this section we will discuss about the process of Gametogenesis in detail:

Gametogenesis in layman terms refer to formation of gametes (an easy guess!!!), well let’s get into deeper, into the  roots and understand it, that actually gametogenesis is the process by which a diploid or a haploid  precursor cell undergoes cell division and maturation to ultimately form a functional gamete.

In plants it’s the haploid cell that forms the gametes but in case of animals it is usually a diploid cell that forms haploid gametes.
Talking specifically of Animal Embryology, Animals produce gametes directly through meiosis in organs called gonads. Males and females of a species that reproduces sexually have different forms of gametogenesis:

• spermatogenesis (male)
• oogenesis (female)

However, before turning into gametogonia, the embryonic development of gametes is the same in males and females
The Common Path

 Gametogonia are usually seen as the initial stage of gametogenesis. However, gametogonia are themselves successors of primordial germ cells. During early embryonic development, primordial germ cells (PGCs) from the dorsal endoderm of the yolk sac migrate along the hindgut to the gonadal ridge. They multiply by mitosis and once they have reached the gonadal ridge in the late embryonic stage, they are called gametogonia. Gametogonia are no longer the same between males and females.