Calf thymus histories comprising two fractions, one rich in lysine, the other having roughly equal amounts of lysine and arginine, Loligo testes histones rich in arginine, and salmine, are compared with respect to their amino acid compositions, and their staining properties when the proteins are fixed on filter paper. The three types of basic proteins; somatic, arginine-rich spermatid histones, and protamine can be distinguished on the following basis. Somatic and testicular histones stain with fast green or bromphenol blue under the same conditions used for specific staining of histones in tissue preparations. The former histones lose most or all of their stainability after deamination or acetylation. Staining of the arginine-rich testicular histones remains relatively unaffected by this treatment. Protamines do not stain with fast green after treatment with hot trichloracetic acid, but are stained by bromphenol blue or eosin after treatment with picric acid. These methods provide a means for the characterization of nuclear basic proteins in situ. Their application to the early developmental stages of Helix aspersa show the following: After fertilization the protamine of the sperm is lost, and is replaced by faintly basic histones which differ from adult histones in their inability to bind fast green, and from protamines, by both their inability to bind eosin, and their weakly positive reaction with bromphenol blue. These "cleavage" histones are found in the male and female pronuclei, the early polar body chromosomes, and the nuclei of the cleaving egg and morula stages. During gastrulation, the histone complement reverts to a type as yet indistinguishable from that of adult somatic cells.

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