The four sentences $( \text{labelled} \; 1, 2, 3, 4)$ below, when properly sequenced would yield a coherent paragraph. Decide on the proper sequencing of the order of the sentences and key in the sequence of the four numbers as your answer:
- In the central nervous systems of other animal species, such a comprehensive regeneration of neurons has not yet been proven beyond doubt.
- Biologists from the University of Bayreuth have discovered a uniquely rapid form of regeneration in injured neurons and their function in the central nervous system of zebrafish.
- They studied the Mauthner cells, which are solely responsible for the escape behaviour of the fish, and previously regarded as incapable of regeneration.
- However, their ability to regenerate crucially depends on the location of the injury.