When a deed in fee is for part only of the land described in a certificate of title the assistant registrar shall also enter a new certificate to the grantor for the part of the land not included in the deed. In every case of transfer the new certificate or certificates shall include all the land described in the original certificate; provided that no new certificate to a grantee of a part only of the land shall be invalid by reason of the failure of the assistant registrar to enter a new certificate to the grantor for the remaining unconveyed portion. In case the land described in a certificate of title is divided into lots, designated by numbers or letters, with measurements of all the bounds, and a plan of the land has been filed with the registrar and verified pursuant to section 501-85; and a certified copy thereof is filed with the assistant registrar bearing the same number as is given to the application, and which plan is filed separately by such number, apart from the registration book containing the original certificate, but which certificate has indorsed thereon a reference to the filed plan, when the registered owner makes a deed or transfer in fee of one or more of such lots, the assistant registrar may, instead of canceling the certificate and entering a new certificate to the grantor for the part of the land not included in the deed of transfer, enter on the original certificate a memorandum of the deed of transfer, with a reference to the lot or lots thereby conveyed as designated on the plan, and a statement that the certificate is canceled as to the lot or lots. Every certificate with the memorandum is as effectual for the purpose of showing the grantor’s title to the remainder of the land not conveyed as if the old certificate had been canceled and a new certificate of the land had been entered. This process may be repeated so long as there is convenient space upon the original certificate for making the memorandum of sale of lots.