Progenesis

Progenesis or gametogenesis is a process of a germ
cells production (ovogenesis for oocytes and spermatogenesis for spermatozoa). These processes take place in male
and female sex glands (ovaries and testes). Spermatogenesis begins in puberty
(13-16 years) and lasts till the old age. Oogenesis begins before the birth
then it is stopped and is renewing in
puberty, and continuous to menopause. In preparation for possible
fertilization, both male and female germ cells undergo a number of changes
involving the chromosomes as well as the cytoplasm. The purpose of these
changes in meiosis is twofold:



1.
Reduction of chromosomes number to
half that in the normal somatic cells, i.e., from 46 to 23. This is
accomplished by two specialized divisions, known as meiotic or maturation
divisions. The reduction in the number of chromosomes is necessary, since
otherwise fusion of a male and female germ cell would result in an individual
with twice number of chromosomes of parent cells.



2.
To alter the shape of germ cells in
preparation for fertilization. The male germ cell, initially large and round,
loses practically all of its cytoplasm and develops a head, neck and tail. The
female germ cell, on the contrary, gradually becomes larger as the result of an
increase in the amount of cytoplasm. At maturity the oocyte has a diameter of
about 120 m.



The proper embryogenesis means
the development of an embryo.
More commonly it occupies all the period of pregnancy. But to be exact, this
period may be subdivided into such stages: the early period of conceptus
(first 7 days), the proper embryonic period (2^th -8^th weeks),
and fetal period (9^th week - to the birth).



Postembryonic
period includes neonatal period, childhood, pubertation, youth, adult, an old
age, which are studied by different sciences, for example: neonatology,
pediatrics, gerontology and so on.



The early
period of conceptus and embryonic period and progenesis are the subjects of a
proper embryologic investigation.



All the
individual development of organism from the very beginning (origin) and to the
death means ontogenesis in opposite to phylogenesis – historical development of some
species.


The interconnection between these two terms was
described at the end of XIX c. in the biogenetic law
of Gekkel-Muller: ontogeny is the short repeating of the phylogeny.