Microfluidic device for preparing and analyzing a biological sample containing biological species, said device comprising: - a single rigid support, - a first unit comprising a first chamber of nonzero volume delimited by walls of the support, a filter separating said first chamber into a first space and a second space, a first channel made in said support opening at one end onto a surface of said support and at another end into said first space and a second channel made in said support and opening at one end onto a surface of said support and at another end into said second space, characterized in that the device has: - a second unit comprising a second chamber made in said rigid support and delimited at least partially by a transparent wall (200, 201) of said support, a third channel made in said support and opening at one end onto a surface of said support and at another end into said second chamber, - a first fluidic transfer channel between the first chamber and the second chamber, made in said support and opening on the one hand into said second chamber and on the other hand at a first bypass node in said second channel, - first flow switching means arranged for selecting connection of the first chamber: - to the exterior only, via the second channel only, by sealing the mouth of the first transfer channel at the level of the first bypass node, or - to the second chamber only, through the first transfer channel, by opening the connection between the second channel and the second transfer channel at the level of the first bypass node. - a single rigid support, - a first unit comprising a first chamber of nonzero volume delimited by walls of the support, a filter separating said first chamber into a first space and a second space, a first channel made in said support opening at one end onto a surface of said support and at another end into said first space and a second channel made in said support and opening at one end onto a surface of said support and at another end into said second space, - a second unit comprising a second chamber made in said rigid support and delimited at least partially by a transparent wall (200, 201) of said support, a third channel made in said support and opening at one end onto a surface of said support and at another end into said second chamber, - a first fluidic transfer channel between the first chamber and the second chamber, made in said support and opening on the one hand into said second chamber and on the other hand at a first bypass node in said second channel, - first flow switching means arranged for selecting connection of the first chamber: - to the exterior only, via the second channel only, by sealing the mouth of the first transfer channel at the level of the first bypass node, or - to the second chamber only, through the first transfer channel, by opening the connection between the second channel and the second transfer channel at the level of the first bypass node. - to the exterior only, via the second channel only, by sealing the mouth of the first transfer channel at the level of the first bypass node, or - to the second chamber only, through the first transfer channel, by opening the connection between the second channel and the second transfer channel at the level of the first bypass node.