1. The inner tiers of cells of the Blastular roof are less pigmented and are known collectively as the nervous layer because they will give rise largely to the neuroblasts of the nervous system

2. The most accepted theory of the cause of Hirschsprung is a defect in the craniocaudal migration of neuroblasts originating from the neural crest that occurs during the first 12 weeks of gestation.

3. In the thoracic segments, located about a third of the way down from the head, certain neuroblasts generate four neurons called the Ap (for "Apterous") cluster, distinguished from other neurons by the expression of a specific set of transcription factors.