AIP Conference Proceedings [AIP Publishing PROCEEDINGS OF THE 4TH INTERNATIONAL SYMPOSIUM ON CURRENT PROGRESS IN MATHEMATICS AND SCIENCES (ISCPMS2018) - Depok, Indonesia (30–31 October 2018)] PROCEEDINGS OF THE 4TH INTERNATIONAL SYMPOSIUM ON CURRENT PROGRESS IN MATHEMATICS AND SCIENCES (ISCPMS2018) - The effect of different concentrations of ethylene glycol and skim milk in vitrification of ovarian rat (Rattus norvegicus L.) strain Sprague-Dawley