An underdeveloped muscel can surely be built up more.
But, the pectoralis major and minor (lower and upper pec) are the only 2 muscles that comprise the pecs. The outer pec is not a muscle, but part of a muscle and is stimulated with every chest exercise to a degree just as the inner pec or upper pec is.
The only thing I beleive you can accentuate on the pecs besides just plain making them bigger is the upper pec since that IS a muscle and does not get worked as hard as the lower on typical flat exercises.
Your outter and inner pec are worked on every chest exercise so that area is unlikely to be underdeveloped.
This is part of the reason only the genetic elite become bodybuilders in the pro sense.
I can point out an excellent example that will back up what I can say since I am not the most techincal speaker.
Look at pictures of Arnold when he first started training at 14 or so. He's not even close to being fully developed, but you can see the beginning of his trademark biceps.
By age 16-17 he's getting pretty thick and now his pecs are filling out. His biceps look the same as before except they are bigger.
By 18-19 he's almost as big and developed and strong as he ever got. His pecs are pretty much fully developed and guess what. His biceps still look the same, only bigger.
Now for anyone that would point out Arnold's own comments on his physique flaws like his legs/calves and triceps well those areas were under developed because he either didn't train them to the same level as his other muscles or those muscles didn''t respond as fast to training as his trademark bi's and pec's. Over time he brought them up to the same elite standard as teh rest of his body, but they did not change shape, they just got bigger. They do appear to have changed shape, but as I said before that's related to the muscle increasing in size in relation to it's attachment points.
In that case he made them bigger, but did not actually emphasize an area of each muscle to make it bigger as muscles really don't work like that.
After the age of 19 or 20 and he had achieved his overall size and shape, he then came to america and refined it. Note his muscle shape does not change otehr than the appearance of change by making it bigger. The most notable physical change in his physique besides the improved legs and triceps was his upper pecs which he was able to increase the size of because the upper pec is an actual muscle and not "part" of a muscle.
Obviously as you train over the years you will look different. ALOT different. AS you muscles grow in relation to their attachment points they ARE gonna look different, but focusing on lower bicep, or outer pec is not gonna change that part of the muscle.
While their have been emg studies on muscles and what exercises seem to work what muscle best, there has never been any proof that you can actually increase the size of say the lower biceps other than making the whole bicep bigger which can give the illusion that the lower bicep got bigger.
Former and first Mr. Universe winner Larry Scott always had amazing lower biceps and I would be willing to bet that they would have been just as amazing regardless of whether he did his trademark preacher curl(which is scientifically proven to be a crap bicep exercise) or regular barbell/dumbell curls.
I really wish I had the gift of speech, but that was never a strong point of mine so these laymen's examples are teh best I can do.
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