Threshold the image to get the blobs, then compute circularities and filter to get those with circularities near 1:
mask = imbinarize(grayImage);
mask = imfill(mask, 'holes');
labeledImage = bwlabel(mask);
props = regionprops(mask, 'Area', 'Perimeter');
allAreas = [props.Area];
allPerimeters = [props.Perimeter];
circularities = allPerimeters .^ 2 ./ (4 * pi * allAreas);
% Get those blobs with circularity near 1.
roundindexes = find(circularities > 0.8 & circularities < 1.5)
% Get new mask
roundBlobs = ismember(labeledImage, roundIndexes);
imshow(roundBlobs);
title('Only the round blobs')
