Complex sentence: He found that thoracic neuroblasts undergo mitotic activity to produce about 100 undifferentiated progeny cells, which accumulate into a discrete packet that remains arrested in an immature state until the onset of metamorphosis, when these cells differentiate into mature neurons.
Split into simpler sentences: He found that thoracic neuroblasts undergo mitotic activity to produce about 100 undifferentiated progeny cells. These cells accumulate into a discrete packet that remains arrested in an immature state until the onset of metamorphosis, when these cells differentiate into mature neurons.