The membrane structural lipids of somatic cells and gonidia isolated from Volvox carteri f. nagariensis spheroids have been characterized. The principal polar lipid components of both cell types are sulfoquinovosyl diglyceride, mono-and digalactosyl diglyceride, phosphatidylglycerol, phosphatidylethanolamine, and 1(3), 2-diacylglyceryl-(3)- O--(N,N,N,-tri- Growth was monitored by filtering 1-ml aliquots ofcell suspension through a 4.5-cm-diameter Millipore gridded filter (No. HAWG04700) and counting the spheroids under a dissecting microscope.Harvest and Disruption of Spheroids. Cultures were harvested by pouring the spheroid-containing medium through a No. HC3-90 nylon mesh screen (Tetko, Inc., Elmsford, N. Y.) having a pore size of 90 um. The concentrated spheroids were washed and resuspended in deionized H20 if intended for lipid extraction.If gonidia and somatic cells were to be isolated, approximately 2 x 105 spheroids in stage 1 of development (see legend of Fig. 2), as determined by phase microscopy, were washed and resuspended in 100 ml cold 0.25 M sucrose in 50 mM Tris (pH 7.4). The suspension was then disrupted in the semimicro chamber of a Waring Blendor, model No. 1120, for 30 s at full power. This treatment released intact gonidia from 85-90%/o of the spheroids.The gonidia and somatic cells (many still embedded in fragments of broken spheroid matrix) were separated from any remaining whole spheroids by passage through the 90-,um pore size nylon screen and concentrated by centrifugation at 365g for 5 min. The pellets were resuspended in 35 ml of the disruption buffer, and 5 ml were loaded onto each of seven discontinuous gradients of sucrose in 50 mM Tris (pH 7.4). Each gradient was composed of 5 ml 0.34 M sucrose, 10 ml 1.0 M sucrose, 10 ml 1.46 M sucrose, and 10 ml 2.0 M sucrose. Centrifugation of the loaded gradients for 10 min at 365g produced three bands of green material. Layer 1, located on top of the 1.0 M sucrose, consisted of large sheets of somatic cells embedded in matrix material. Layer 2, located on top of the 1.46 M sucrose, was composed of single somatic cells and small sheets of matrix containing somatic cells. Layer 3, lying above the 2.0 M sucrose, consisted of gonidia, contaminated by a very occasional large sheet of somatic cellcontaining matrix. The purity of the isolated fractions is illustrated in Figure 1.Lipid Extraction and Analysis. Spheroids or isolated cell types were resuspended in a minimum volume ofwater or cell disruption medium, and lipids were extracted by the procedure of Bligh and Dyer (4). Water-soluble impurities were removed by washing the organic phase with a simulated Folch upper phase (10).When a further resolution of the bulk lipid mixture was desired, the washed lipid extract was chromatographed on silicic acid (100 mesh, Mallinckrodt), eluting NL with CHC13, glycolipids with acetone, and PL with CHCl3-methanol (1:1, v/v). TLC of polar lipids was performed, except where noted, on Silica Gel G plates, using the solvent system CHC13-acetic acid-meth...