Complex sentence: The fetal pig develops primary teeth (which are later replaced with permanent teeth) and some may erupt during fetal stage, which is why some of the pigs that are/will be dissected show evidence of teeth.
Split into simpler sentences: The fetal pig develops primary teeth (which are later replaced with permanent teeth). Some may erupt during fetal stage, which is why some of the pigs that are/will be dissected show evidence of teeth.