Humans begin their existence as a single undifferentiated cell and proceed to become a highly complex and organized system consisting of specialized cells (liver, skin, brain, blood, etc.). This change BEST exemplifies the principle.

Orthogenetic

Explanation:

The Orthogenetic principle of development was put forward by Heinz Werner, in the year 1975.It is a biological principle of development. This principle of development states that all the aspects of the functioning of a living organism develop progressively from a less differentiated state to a more differentiated state. Besides differentiation, there is also hierarchic integration and articulation of functioning such as perception, cognition, etc. This leads to the development of more complex forms from a simple form.Therefore, a single undifferentiated cell giving rise to the complex organized system is an example of the orthogenetic principle.

answer: - b cells

explanation: -

there are many types of white blood cells present in our body. one such class of white blood cells are the lymphocytes. the b cells belong to the lymphocyte class of wbcs. the b cells perform their action against the pathogen by producing specialised proteins called as antibodies. these antibodies are produced as and when the b cells encounter pathogens in response to the antigens present on the surface of the pathogen. these antibodies bind to the antigens present on the pathogen and then, target them for destruction by other cells of the immune system.

so, the b cells mediate their action by identifying the antigens present on the surface of the pathogens and then producing antibodies in return to mediate its effect.