How to make a voxel terrain generate all around the start point

I have a voxel terrain generator, the problem is that it only generates in the positive x + Z position. Is there a way to also make it generate in the negative X and Z directions ? Here is the code.

 using UnityEngine;
 using System.Collections;
  
 public class World : MonoBehaviour
 {
   
     public GameObject chunk;
     public Chunk[,,] chunks;  //Changed from public GameObject[,,] chunks;
     public int chunkSize = 16;
     public byte[,,] data;
     public int worldX = 16;
     public int worldY = 16;
     public int worldZ = 16;
   
     // Use this for initialization
     void Start ()
     {
   
         data = new byte[worldX, worldY, worldZ];
    
         for (int x=0; x<worldX; x++) {
             for (int z=0; z<worldZ; z++) {
                 int stone = PerlinNoise (x, 0, z, 10, 3, 1.2f);    //PerlinNoise (int x, int y, int z, float scale, float height, float power)
                 stone += PerlinNoise (x, 300, z, 200, 4, 0) + 50;
                 int dirt = PerlinNoise (x, 1000, z, 50, 20, 0) + 1;
      
                 for (int y=0; y<worldY; y++) {
                     if (y <= stone) {
                         data [x, y, z] = 1;
                     } else if (y <= dirt + stone) {
                         data [x, y, z] = 2;
                     }
       
                 }
             }
         }
    
    
         chunks = new Chunk[Mathf.FloorToInt (worldX / chunkSize), Mathf.FloorToInt (worldY / chunkSize), Mathf.FloorToInt (worldZ / chunkSize)];
         
    
     }
     
     public void GenColumn(int x, int z){
         for (int y=0; y<chunks.GetLength(1); y++) {
       
                     //Create a temporary Gameobject for the new chunk instead of using chunks[x,y,z]
                     GameObject newChunk = Instantiate (chunk, new Vector3 (x * chunkSize - 0.5f,
  y * chunkSize + 0.5f, z * chunkSize - 0.5f), new Quaternion (0, 0, 0, 0)) as GameObject;
       
                     chunks [x, y, z] = newChunk.GetComponent ("Chunk") as Chunk;
                     chunks [x, y, z].worldGO = gameObject;
                     chunks [x, y, z].chunkSize = chunkSize;
                     chunks [x, y, z].chunkX = x * chunkSize;
                     chunks [x, y, z].chunkY = y * chunkSize;
                     chunks [x, y, z].chunkZ = z * chunkSize;
       
                 
             }
     }
     
     public void UnloadColumn(int x, int z){
         for (int y=0; y<chunks.GetLength(1); y++) {
             Object.Destroy(chunks [x, y, z].gameObject);
             
         }
     }
   
     int PerlinNoise (int x, int y, int z, float scale, float height, float power)
     {
         float rValue;
         rValue = Noise.GetNoise (((double)x) / scale, ((double)y) / scale, ((double)z) / scale);
         rValue *= height;
    
         if (power != 0) {
             rValue = Mathf.Pow (rValue, power);
         }
    
         return (int)rValue;
     }
   
   
     // Update is called once per frame
     void Update ()
     {
   
     }
   
     public byte Block (int x, int y, int z)
     {
    
         if (x >= worldX || x < 0 || y >= worldY || y < 0 || z >= worldZ || z < 0) {
             return (byte)1;
         }
    
         return data [x, y, z];
     }
 }