Kovalevsky V. A. (1845-1916) - an outstanding Russian anatomist and histologist. He formulated and proposed the so-called germ layer theory. Thanks to this theory, it became clear how tissues and organs are formed during the development of an embryo.
In modern science, it is generally accepted that initially there is only one layer - the three-layer mesodermal layer. From it the notochord (the rudiment of the spine), the neural tube and the coelomium, which lies in the center of the anlage of the internal organs, are formed.
It is believed that over time, the two-layer nature of the first layer causes it to divide