Chromatids in metaphase?
The chromosomes in your picture do have sister chromatids, they are just very close together. A chromosome needs to pass through replication before it can compact into the typical metaphase chromosome shape depicted above. When I prepare metaphase chromosomes, I usually see a mixture of chromosome sets where the chromatids are close together (like above) and clearly separated. I think this is related to the time each metaphase has spent in the colcemid block-the longer the cell has been blocked at metaphase for, the longer the enzymes that work to separate the two chromatids have acted on it. Also chromosomes keep getting more and more compacted during the colcemid block-if you leave them for too long, they become a lot smaller.  This paper has very cool pictures showing the links between the sister chromatids:
http://www.ncbi.nlm.nih.gov/pubmed/18461479