Creating PLane Mesh via code

Hello,
Im trying to learn how to create a plane mesh via code.
so far i got this.

"Im trying to learn how to create plane meshes via code. I was able to draw my vertices and and half my triangle.

"using UnityEngine;
using System.Collections;

public class DrawPlaneMesh : MonoBehaviour
{
private Mesh myMesh;
private Vector3[] newVertices;
private int[] newTriangles;


public int xSize;
public int zSize;

public float seperationX;
public float seperationZ;


private void Start()
{

newVertices = new Vector3[xSize * zSize];
newTriangles = new int[(xSize - 1) * (zSize - 1) * 6];
myMesh = new Mesh();
gameObject.AddComponent<MeshFilter>();
gameObject.AddComponent<MeshRenderer>();


int counter = 0;
for(int i = 0; i < xSize; i++){
for(int j = 0; j < zSize; j++){

GenerateSphere(i * seperationX,0,j * seperationZ);

int count = j+zSize * i;
newVertices[ConvertToArrayIndex(i,j,xSize)] = new Vector3(i * seperationX,0,j * seperationZ);

Debug.Log(ConvertToArrayIndex(i,j,xSize ));
Debug.Log(count);

if((i < xSize-1) && j < (zSize-1)) {


newTriangles[counter] = ConvertToArrayIndex(i,j,xSize);
counter++;
newTriangles[counter] = ConvertToArrayIndex(i + 1,j,xSize);
counter++;
newTriangles[counter] = ConvertToArrayIndex(i,j + 1,xSize);
counter++;
newTriangles[counter] = ConvertToArrayIndex(i,j,xSize);
counter++;
newTriangles[counter] = ConvertToArrayIndex(i + 1,j,xSize);
counter++;
newTriangles[counter] = ConvertToArrayIndex(i,j + 1,xSize);
counter++;

}
}
}





myMesh.vertices = newVertices;
myMesh.triangles = newTriangles;
GetComponent<MeshFilter>().mesh = myMesh;
}


void GenerateSphere(float x,float y,float z){
GameObject sphere = GameObject.CreatePrimitive(PrimitiveType.Sphere);
sphere.transform.localPosition = new Vector3(x,y,z);
sphere.transform.localScale = new Vector3(0.1f,0.1f,0.1f) ;

}

int ConvertToArrayIndex(int i, int j, int size){
return j+size * i;

}
}"

this makes the vertices and triangles. but the triangles are only half.
Meaning that
Triangles go
Lets say if im gonna make a smal plane with 0,1,2,3
The triangle is only showing from 0,1,2.
How can i make it so it also shows the 1,2,3 triangle?