Off By One error generating a mesh

I have what I think is an off-by-one error in my mesh generation code but can seem to find it after staring at it for hours. The function is below and I've attached the _HeightMapPreview texture and a screen-shot of the issue. The _mapSize is 512. The issue is there is a gap between each block of the terrain. I'm hoping another set of eyes (or a few sets) will help.

Edit: The Heightmap has an alpha channel so it looks blank.

Code (csharp):

1.  
2.    public void BuildMesh()
3.     {
4.         GameObject oTerrain = new GameObject();
5.         oTerrain.name = "Terrain";
6.        
7.         float meshSize = 200f;
8.         if (_mapSize == 256)
9.             meshSize = 400;
10.         if (_mapSize == 512)
11.             meshSize = 800;
12.        
13.         int blocks = (int) Mathf.Ceil(_mapSize / 128f);
14.         int partsPerBlock = (int)Mathf.Ceil(_mapSize / blocks);
15.        
16.         Debug.Log("Map Size: " + _mapSize.ToString());
17.         Debug.Log("Blocks: " + blocks.ToString());
18.         Debug.Log("Parts Per Block: " + partsPerBlock.ToString());
19.        
20.         for (int i = 0; i < blocks; i++)
21.         {
22.             for (int j = 0; j < blocks; j++)
23.             {
24.                 GameObject oBlock = new GameObject();
25.                 oBlock.name = "Block " + (j*blocks + i).ToString();
26.                 oBlock.transform.parent = oTerrain.transform;
27.                 oBlock.AddComponent<MeshFilter>();
28.                 oBlock.AddComponent<MeshRenderer>();
29.                
30.                 Mesh oMesh = oBlock.GetComponent<MeshFilter>().sharedMesh;
31.                 if(oMesh == null)
32.                     oMesh = new Mesh();
33.            
34.                 Vector3[] vertices = new Vector3[partsPerBlock * partsPerBlock];
35.                 Vector2[] uv = new Vector2[partsPerBlock * partsPerBlock];
36.                 Vector4[] tangents = new Vector4[partsPerBlock * partsPerBlock];
37.                
38.                 Vector2 uvScale = new Vector2 (1.0f / (_mapSize - 1), 1.0f / (_mapSize - 1));
39.                 Vector3 sizeScale = new Vector3(meshSize / (_mapSize - 1), 50f, meshSize / (_mapSize - 1));
40.                
41.                 int startX = partsPerBlock * i;
42.                 if (startX < 0)
43.                     startX = 0;
44.                 int endX = startX + partsPerBlock;
45.                 if (endX > _mapSize)
46.                     endX = _mapSize;
47.                 int startY = partsPerBlock * j;
48.                 if (startY < 0)
49.                     startY = 0;
50.                 int endY = startY + partsPerBlock;
51.                 if (endY > _mapSize)
52.                     endY = _mapSize;
53.                
54.                 for (int x = startX; x < endX; x++)
55.                 {
56.                     for (int y = startY; y < endY; y++)
57.                     {
58.                         int posX = x - startX;
59.                         int posY = y - startY;
60.                         //Debug.Log("["+posX.ToString()+", "+posY.ToString()+"] - "+(posY * partsPerBlock + posX).ToString() + " - "+vertices.Length.ToString());
61.                        
62.                         float h = _HeightMapPreview.GetPixel(x,y).r +_HeightMapPreview.GetPixel(x,y).g + _HeightMapPreview.GetPixel(x,y).b + _HeightMapPreview.GetPixel(x,y).a;
63.                         Vector3 vertex = new Vector3(x, h, y);
64.                         vertices[posY * partsPerBlock + posX] = Vector3.Scale(sizeScale, vertex);
65.                         uv[posY * partsPerBlock + posX] = Vector2.Scale(new Vector2 (x, y), uvScale);
66.        
67.                         // Calculate tangent vector: a vector that goes from previous vertex
68.                         // to next along X direction. We need tangents if we intend to
69.                         // use bumpmap shaders on the mesh.
70.                         h = _HeightMapPreview.GetPixel(x-1,y).r +_HeightMapPreview.GetPixel(x-1,y).g + _HeightMapPreview.GetPixel(x-1,y).b + _HeightMapPreview.GetPixel(x-1,y).a;
71.                         Vector3 vertexL = new Vector3(x - 1, h, y);
72.                         h = _HeightMapPreview.GetPixel(x+1,y).r +_HeightMapPreview.GetPixel(x+1,y).g + _HeightMapPreview.GetPixel(x+1,y).b + _HeightMapPreview.GetPixel(x+1,y).a;
73.                         Vector3 vertexR = new Vector3(x + 1, h, y);
74.                         Vector3 tan = Vector3.Scale(sizeScale, vertexR - vertexL).normalized;
75.                         tangents[posY * partsPerBlock + posX] = new Vector4(tan.x, tan.y, tan.z, -1.0f);
76.                     }
77.                 }
78.                
79.                 // Assign them to the mesh
80.                 oMesh.vertices = vertices;
81.                 oMesh.uv = uv;
82.            
83.                 // Build triangle indices: 3 indices into vertex array for each triangle
84.                 int[] triangles = new int[(partsPerBlock) * (partsPerBlock) * 6];
85.                 int index = 0;
86.                 for (int y = startY; y < endY - 1; y++)
87.                 {
88.                     for (int x = startX; x < endX - 1; x++)
89.                     {
90.                         int posX = x - startX;
91.                         int posY = y - startY;
92.  
93.                         // For each grid cell output two triangles
94.                         triangles[index++] = (posY * partsPerBlock) + posX;
95.                         triangles[index++] = ((posY+1) * partsPerBlock) + posX;
96.                         triangles[index++] = (posY * partsPerBlock) + posX + 1;
97.                        
98.                         triangles[index++] = ((posY+1) * partsPerBlock) + posX;
99.                         triangles[index++] = ((posY+1) * partsPerBlock) + posX + 1;
100.                         triangles[index++] = (posY * partsPerBlock) + posX + 1;
101.                     }
102.                 }
103.                
104.                 // And assign them to the mesh
105.                 oMesh.triangles = triangles;
106.                
107.                 // Auto-calculate vertex normals from the mesh
108.                 oMesh.RecalculateNormals();
109.                
110.                 // Assign tangents after recalculating normals
111.                 oMesh.tangents = tangents;
112.                
113.                 oBlock.GetComponent<MeshFilter>().sharedMesh = oMesh;
114.             }
115.         }
116.     }
117.