The pulp of the spleen contains lymphocytes and serves to fight pathogens, whereas the pulp of the spleen contains sinusoids and filters blood.

The given blanks can be filled with white pulp and red pulp.

Explanation:

In the lymphatic system, the spleen is considered as the largest organ. It is situated in the upper left part of the abdominal cavity, the main function of the spleen is to filter blood containing the destructed cells, pathogens like viruses and bacteria, and cellular debris.  

The spleen generates and develops the mature immune cells, which possess the tendency of finding and eliminating the pathogens. The white pulp of the spleen comprises the immune cells, that is, the B and T lymphocytes, which help in fighting against the pathogens.  

On the other hand, the red pulp of the spleen comprises connective tissue called the cords of Billroth, and various splenic sinusoids, which are filled with blood, providing it a red appearance. Its main function is to filter the blood containing microbes, pathogens, and worn-out or the defective RBCs.  

not necessarily. higher fitness requires the organism to be reproductive successful. for example, an organism at age 10 with 15 offspring is much more fit than an organism at age 15 with only 5 offspring.