The teachings of Charles Darwin (1809–1882) on the development of living beings and his descending theory, theses of which were documented with newer and newer materials and evidence gleaned from the field of embryology, were extremely important for the progress and success of embryological research , as well as for increasing interest in embryology. , comparative anatomy and paleontology.
This theory was further developed by a number of modern researchers , among which should be mentioned primarily Ernest Haeckel (1834-1919), who discovered the basic biogenetic law, a follower of Darwin and an enthusiastic propagandist of his teachings.
Among the many Russian embryologists of that time, the most prominent were I. I. Mechnikov (1845-1916) and A. O. Kovalevsky (1840-1901). Both scientists with their outstanding works in comparative embryology contributed significantly to the development of this science.
Among the Czech researchers during this period was Jan Evangelist Purkinje (1787–1869), who was known as a world-class scientist mainly due to his work on histology and physiology; in addition, he was also engaged in embryological studies of the bird’s egg (he discovered the egg core) and studied the development of various organs.
As shown by the results of experimental embryology (developmental mechanics), in some animals (for example, vertebrates), separate blastomeres can be separated from each other in a fragmented egg, and a new organism arises from each cell so separated. Therefore, the blastomeres of these eggs are “totipotent”, they have a great development potential, as well as a similar egg as a whole.
The ooplasm of such eggs and blastomeres has a plastic character, due to which the defect caused by the separation of the blastomeres from each other is compensated, regulated. In this regard, eggs with these properties are called regulatory eggs. The possible future development of regulatory eggs and their parts, that is, the so-called prospective potency, is of great importance.
Of the individual parts ( blastomeres ) of such eggs with a large prospective potency, much more organs and tissues can arise than develop from them under normal conditions, when they do not separate from each other. In other words, more of the various components of the organism can develop from them than this is included in their normally carried out development; this is the so-called prospective value. In short, the prospective potency of regulatory eggs is much greater than their prospective value (G. Drish).
In contrast to regulatory eggs , such eggs are known that in the stage of several blastomeres are already so differentiated that if these blastomeres are separated from each other, only a certain part of the body arises from each of them, and not the whole organism as a whole in regulatory eggs. In all likelihood, tissue differentiation of blastomeres is already occurring in them early, and this subsequently does not allow their protoplasm to regulate the defect that has arisen. The prospective potency of these so-called mosaic eggs and blastomeres is in varying degrees equal to their prospective value.
Based on these data , both of the above development theories have been updated, but in a new form. Proponents of neo-evolutionary theory, who advocate preformist tendencies, referring to mosaic eggs, consider them proof of the existence of preformed “organ-forming” areas in the ooplasm of an egg, or rather, in blastomeres, that is, areas that have a priori deterministic development.
On the contrary, neoepigenetics emphasize the equipotency of all areas of ooplasm and blastomeres, which in the process of development under the influence of external and internal conditions can differentiate in a certain direction and even mutually replace each other.
At present, development is understood not only as a direct reproduction of the sequence of similar organisms and their components, but also as a long chain of qualitative leaps and changes due to which mutually dissimilar or even slightly different components are reproduced in the process of development .
Consequently, the direction of phylogenetic development is not predetermined, preformed, it is the result of changes in the heredity of the organism, changes that are adequate to the effects of environmental conditions assimilated by the living organism over a long period of phylogenetic development.