There are two contrasting theories of embryogenesis in modern embryology,

There are two contrasting theories
of embryogenesis in modern embryology, which describe the human
development– preformation and epigenesis.

The basic
point of preformation theory is the next: from the very beginning of the
embryogenesis the whole body of the human being is presented in embryo and
after fertilization his little organs can enlarge. There are two trends in this
theory. Spermatics support Hippocrat, who suppose the spermatozoon been
principle material of the future human body and oocyte only gives nutrition to
this. Their opponents – ovists – consider the oocyte contains the main
structures of human being and the biologic meaning of spermatozoa is to
activate them in definite way. This theory was restored and especially
widespread in XVII-XVIII c.c.

The
XVIII-XIX c.c. was the period of an active “struggle” between the different
points of scientific wiev on the human embryogenesis. In 1775 Lazano
Spallanzani showed that both the ovum and sperm are necessary for new
individual development. At the middle of XIX c. molecular biology had begun to
develop. The structure and functions of DNA were described and in 1759 Caspar
Wolff had grounded the theory of postformation or epigenesis, which principal
ideas were firstly created by Aristotel many centuries ago. The main points of
this theory are:

1.
Development of new organism begins
after the merging of oocyte and spermatozoon.

2.
Characteristic set (collection0 of
chromosomes is renewed.

3.
Genetic information begins its
realization.

Heinrich
Christian Pander later had described three germ layers of embryo (blastoderm),
from which different structures appear.

Active
development of medicine and embryologic investigations of the past hundred
years have demonstrated an epigenetic nature of embryo development. The
fertilized egg, possessing a simple form and exhibiting an apparently
undifferentiated structure, undergoes a series of developmental changes, which
result in the spatial differentiation of the mature organism with its specialized
types of cells, tissues and organs. But this differentiation is directly
connected with genes, which are present in the chromosomes of zygote nucleus.
So, special features of human body in future depend on the heredity information
of oocyte and spermatozoon, which participate in fertilization.

There are some principle differences
between germ cells and somatic ones. First of all, germ cells contain 23
chromosomes, they have possibility to assimilation and without fertilization
they die in a short period of time. Somatic cells have 46 chromosomes, they can
not merge but they are still alive without fertilization.

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