Through the miacles of medicine and technology researchers have had some success with growing and tranplanting hollow, relatinlely simple organs such as tracheas abd bladders. Building organs like the heart heary and lungs have proved to be much more difficult

Explanation:

Simple organs such as the bladder are basically a bag with three entries, this makes these organs easier to create successfully. Other organs such as the heart are much more complex since they contain various Chambers, Valves, Vessels, Walls, and arteries, all of which serve a purpose. Every single part of these organs must be exact in order for it to function correctly, thus making it extremely difficult to recreate. You can see the attached picture of a heart below to get an idea of how complex it actually is.

the cladogram shown here shows similarities and differences among several classes of vertebrates. anatomically, the four have limb structure as their common ancestor. we might separate the mammals from the other three classes because they a) divergent; do not lay eggs b) identical; are warm-blooded c) varied; a four chambered heart d) homologous; have mammary glands

the answer you are looking for is the united states forest service.

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