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Two fractions of thylakoid membranes (TMF) have been isolated from disrupted (French press) algal cells by using a discontinuous sucrose gradient . TMF-II consists mostly of thylakoid membranes still partially organized in grana ; it contains also fragments of chloroplast envelope, pyrenoid tubules, and starch granules ; thus it amounts to a fraction of chloroplast fragments which have lost practically all matrix components . TMF-I consists of smaller chloroplast fragments and is contaminated to a larger extent than TMF-II by other subcellular components, primarily mitochondria. TMF-II accounts for about 12% of the protein and 30 % of the chlorophyll of the whole cell ; it contains cytochrome 554 and carotenoids in the same ratio to chlorophyll as the latter, and shows photosystems I and II activities but lacks enzymatic activities characteristic of the dark reactions . During the greening of the y-1 mutant of Chlamydomonas, TMF's have been isolated over a range of chlorophyll concentrations from 5 to 25 µg/10 7 cells . The results showed that during this period the ratios of chlorophyll to cytochrome 554 and of chlorophyll to carotenoids, and the relative concentrations of individual carotenoids were continuously changing . The findings support the view that during greening, thylakoid membranes are produced by multistep assembly.

Activation of unfertilized Chaetopterus eggs by treatment with an excess of KCl may lead to the production of unicellular ciliated larvae (Lillie's differentiation without cleavage). The effects of a number of inhibitors of protein (puromycin, cycloheximide, emetin), RNA (actinomycin D), and DNA (hydroxyurea) synthesis of differentiation without cleavage and on normal development have been studied in Chaetopterus. Incorporation of radioactive leucine, uridine, and thymine has been followed by biochemical methods and by autoradiography. The DNA content of the large polyploid nucleus has been estimated by cytophotometry. The initial pseudocleavage period of differentiation without cleavage is characterized by a burst in DNA and protein synthesis; the inhibitors have little or no effect on this burst and on pseudocleavage itself. Protein and DNA synthesis levels off during the following phase (segregation), but the inhibitors become more effective. RNA synthesis is almost linear for 20 h. These results are compared with those obtained on eggs from other species.

SUMMAR Y Aphidicolin, an inhibitor ofDNA polymerase a. arrests DNA synthesis without affecting RNA and protein synthesis at ail stages of sea urchin development. Cleavage is quickly stopped and hatching is prevented by the présence of the drug at 2 /u,g/ml. Treatment with aphidicolin of young blastulae prevents gastrulation. but inhibition of archenteron invagination and skeleton formation is incom plète when the drug is added to late blastulae or early gastrulae. The rôle of cell division in gastrulation is discussed.

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