In this paper we consider the closure of me="mml6">b=MathURL&_method=retrieve&_udi=B6WH2-4GG8W62-3&_mathId=mml6&_user=10&_cdi=6838&_rdoc=8&_handle=V-WA-A-W-WU-MsSAYVW-UUA-U-AABZWYBDWW-AABBYZVCWW-CADVYBEYA-WU-U&_acct=C000050221&_version=1&_userid=10&md5=ce73dc7a5669ca417332af22616de1f2">mg src="http://www.sciencedirect.com/cache/MiamiImageURL/B6WH2-4GG8W62-3-8/0?wchp=dGLbVtz-zSkWb" alt="Click to view the MathML source" align="absbottom" border="0" height=12 width=11> and me="mml7">b=MathURL&_method=retrieve&_udi=B6WH2-4GG8W62-3&_mathId=mml7&_user=10&_cdi=6838&_rdoc=8&_handle=V-WA-A-W-WU-MsSAYVW-UUA-U-AABZWYBDWW-AABBYZVCWW-CADVYBEYA-WU-U&_acct=C000050221&_version=1&_userid=10&md5=25c692c3c3d5aa20611cb71245b1b63b">mg src="http://www.sciencedirect.com/cache/MiamiImageURL/B6WH2-4GG8W62-3-9/0?wchp=dGLbVtz-zSkWb" alt="Click to view the MathML source" align="absbottom" border="0" height=15 width=17> under elementary equivalence, direct sums and full Cartesian products, homomorphic images, and residual properties. We further show that every metabelian group G which has a central order can be embedded in a metabelian group me="mml8">b=MathURL&_method=retrieve&_udi=B6WH2-4GG8W62-3&_mathId=mml8&_user=10&_cdi=6838&_rdoc=8&_handle=V-WA-A-W-WU-MsSAYVW-UUA-U-AABZWYBDWW-AABBYZVCWW-CADVYBEYA-WU-U&_acct=C000050221&_version=1&_userid=10&md5=0f1665c1db2cd7b29d5c025a93ca29f9">mg src="http://www.sciencedirect.com/cache/MiamiImageURL/B6WH2-4GG8W62-3-B/0?wchp=dGLbVtz-zSkWb" alt="Click to view the MathML source" align="absbottom" border="0" height=14 width=13> belonging to me="mml9">b=MathURL&_method=retrieve&_udi=B6WH2-4GG8W62-3&_mathId=mml9&_user=10&_cdi=6838&_rdoc=8&_handle=V-WA-A-W-WU-MsSAYVW-UUA-U-AABZWYBDWW-AABBYZVCWW-CADVYBEYA-WU-U&_acct=C000050221&_version=1&_userid=10&md5=3786496ec5631ec3b935365ad2bd5da2">mg src="http://www.sciencedirect.com/cache/MiamiImageURL/B6WH2-4GG8W62-3-C/0?wchp=dGLbVtz-zSkWb" alt="Click to view the MathML source" align="absbottom" border="0" height=12 width=11> so that each central order on G can be extended to me="mml10">b=MathURL&_method=retrieve&_udi=B6WH2-4GG8W62-3&_mathId=mml10&_user=10&_cdi=6838&_rdoc=8&_handle=V-WA-A-W-WU-MsSAYVW-UUA-U-AABZWYBDWW-AABBYZVCWW-CADVYBEYA-WU-U&_acct=C000050221&_version=1&_userid=10&md5=da52543d9f7a79d23df920facb26a23a">mg src="http://www.sciencedirect.com/cache/MiamiImageURL/B6WH2-4GG8W62-3-2/0?wchp=dGLbVtz-zSkWb" alt="Click to view the MathML source" align="absbottom" border="0" height=14 width=13>. This is achieved by ascending HNN-extensions. We further show that a finitely generated metabelian group has a central order if and only if it is residually torsion-free nilpotent.