grow
Explanation
This comic takes place in what appears to be a scientific laboratory.
A scientist excitedly announces: "I don't believe it! It's against nature's order! We shouldn't be able to manufacture human reproductive organs in vitro!"
His colleague responds: "Oh, grow some balls!"
The joke is a perfect double entendre. The phrase "grow some balls" is a common colloquial expression meaning "be brave" or "stop being a coward" -- it's used to tell someone to toughen up and stop being squeamish or hesitant. However, in the context of a laboratory that has literally just achieved the ability to grow human reproductive organs in vitro (in a lab setting), "grow some balls" is also a completely literal instruction -- as in, use the new technology to grow testicles.
The first scientist is expressing moral or scientific shock at the breakthrough, suggesting perhaps they shouldn't have this capability. His colleague's response works simultaneously as a dismissal of his ethical concerns ("stop being a coward about it") and as a practical suggestion for what to do with the technology ("manufacture testicles").
The comic plays on the intersection of scientific ethics and crude idiom, which is a frequent SMBC theme. It also touches on the broader debate about the ethics of growing human organs in laboratories -- a real and active area of bioethics -- but deflates the seriousness with a pun.