L'inglese mi sembra molto zoppicante. L'idea che mi viene è questa, nel senso che la ghiandola mammaria si sviluppa essenzialmente a partire dalla pubertà.

Questa evoluzione inizia subito dopo il concepimento, ma bisognerà attendere l'inizio della pubertà perché si avviino le modificazioni sessuali proprie del seno femminile. La ghiandola mammaria femminile esprime infatti la sua massima attività di crescita durante la pubertà e raggiunge la sua completa maturazione nell'età adulta, dopo la gravidanza e l'allattamento.
http://it.health.yahoo.net/c_special.asp?id=10086&s=3&c=0

The sentence makes no real sense - it is supposed to be talking of breast anatomy and the English word latecomer makes no sense at all. I imagine this is a translation from another language

However, as the breast is made up, from the gross anatomy point of view, of milk (lactiferous) ducts, lactiferous sinuses (which modern techniques have shown us do not in fact exist), glandular tissue, adipose (fatty) tissue, connective tissue and chest muscles, I am presuming that the term "latecomer" is some sort of terrible mistake for milk ducts.

For information on breast anatomy see:
http://www.007b.com/breast_anatomy.php
http://it.wikipedia.org/wiki/File:Breast_anatomy_normal_sche...

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Note added at 6 hrs (2009-08-08 22:09:37 GMT)
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The only part of the breast remotely similar is the milk duct, which is called a conduit in other languages. You really need to check with whoever gave you the translation to do.

Of course, they have also left out one very important part of the breast - the nipple, but that sounds even less like your term.