▁situ ▁when ▁m align ant ▁cells ▁are ▁present ▁as ▁a ▁tum or ▁but ▁have ▁not ▁met ast as ized , ▁or ▁inv aded ▁beyond ▁the ▁layer ▁or ▁t issue ▁type ▁where ▁it ▁arose . ▁For ▁example , ▁a ▁cancer ▁of ▁ep ith el ial ▁origin ▁with ▁such ▁features ▁is ▁called ▁ ▁car cin oma ▁in ▁situ , ▁and ▁is ▁defined ▁as ▁not ▁having ▁inv aded ▁beyond ▁the ▁bas ement ▁memb rane . ▁In ▁contrast , ▁an ▁invas ive ▁car cin oma ▁has ▁inv aded ▁beyond ▁the ▁bas ement ▁memb rane . ▁ ▁Con clus ions ▁The ▁significance ▁of ▁studies ▁of ▁the ▁morph ological ▁manifest ations ▁and ▁mole cular ▁gen etic ▁mechan isms ▁of ▁the ▁invasion ▁and ▁met ast asis ▁of ▁m align ant ▁tum ors ▁is ▁not ▁in ▁doubt . ▁The ▁results ▁of ▁numerous ▁studies ▁clearly ▁demonstrate ▁that ▁migration ▁of ▁tum or ▁cells ▁during ▁invas ive ▁growth ▁can ▁occur ▁both ▁via ▁single ▁cells ▁and ▁via ▁groups ▁of ▁cells . ▁This ▁divers ity ▁of ▁cell ▁migration ▁types ▁probably ▁leads ▁to ▁the ▁development ▁of ▁intr atum oral ▁heter ogene ity ▁that ▁is ▁represented , ▁e . g . ▁in ▁breast ▁cancer , ▁by ▁different ▁morph ological ▁structures : ▁al ve olar , ▁trab ec ular , ▁and ▁solid ▁structures ▁and ▁discrete ▁groups ▁of ▁tum or ▁cells . ▁A ▁number ▁of ▁bio chem ical ▁and ▁mole cular ▁gen etic ▁mechan isms ▁are ▁known ▁that ▁enable ▁m align ant ▁cells ▁to ▁in va de ▁surrounding ▁t issues ▁and ▁gain ▁the ▁ability ▁to ▁spread ▁far ▁beyond ▁the ▁primary