Answer:
Answer is Sinoatrial node, SA.
Explanation:
The sinoatrial node, SA, is known to be a group of cells which are found at the top of the heart.
The electrical signal that is initiated by the sinoatrial node bring about or stimulates the atria and then the ventricle of the heart.
It should be noted that, the sinoatrial node are regarded as the pacemaker cells.
If you look at the taxonomic classification there are 7 levels and they are 1. Kingdom; 2. Phylum; 3. Class; 4. Order; 5. Family; 6. Genus and 7. Species.
The taxonomic level phylum includes greatest number of species for a given genus.
Pre-embryonic cleavages make use of the abundant cytoplasm of the conceptus as the cells rapidly divide without changing the total volume.