ALBERT

Notizen: [Auszug] As is well known, the speed and simplicity of the mercury vapour condensation pump has led to its almost universal adoption in the production of extremely high vacua. It suffers from the disadvantage that, while it rapidly removes gases and vapours from the vessel to be exhausted, it does not ...

Notizen: [Auszug] THE phenomenon described by Prof. A. W. C. Menzies and Mr. C. A. Sloat in NATURE for Mar. 9, p. 348, can, I think, be explained from some results I obtained in 1925 in connexion with the banded crystallisation of sulphur films. The inside of a test-tube was covered with a film of molten ...

Notizen: [Auszug] THE recent work of Millikan (Science, 62, 444, 1925) has added to our list of radiations, some of far shorter wave-length than any hitherto discovered. By extrapolation of A. H. Compton's formula for absorption (Phys. Rev., 21, 483, 1923), it has been estimated that these new rays have a range ...

Notizen: Summary 1. The germ cells of the hermaphrodite embryos ofIcerya purchasi contain the diploid number of chromosomes (four) from the time of the first segregation of the germ cells up to the time of the emergence of the nymphs from the eggs. 2. During the early part of the first nymphal instar, usually before the attachment of the nymph to the host plant has occurred, haploid nuclei appear among the diploid nuclei of the gonad. 3. The chromosomes of these haploid nuclei are recognisable by size and shape as comprising one member of each pair of the diploid chromosome complex. It follows that a reduction from the diploid to the haploid condition has taken place, but no evidence on the cytological mechanism underlying this reduction is available. 4. The haploid nuclei usually give rise to the central core of the gonad and often to a variable amount of more or less isolated lateral masses of tissue. From this haploid tissue the spermatozoa of the hermaphrodite gonad are derived. 5. In the spermatogenesis only one meiotic division occurs. This is equational in character. The processes of spermatogenesis correspond exactly with those described for the haploid males of this species. 6. In two aberrant cases spermatids were produced from diploid cells. Two divisions were undergone and reduction to the haploid state accomplished, although without synapsis. Cytology of these aberrant cases is incomplete. 7. The diploid nuclei of the gonad proliferate slowly throughout the male phase, and usually come to form the outer or peripheral zone of the young gonad. The diploid nuclei give rise to the follicular epithelium of the gonad, the egg cells, the nurse cells, and to the greater part of the interstitial tissue of the gonad. 8. Oogonia are differentiated from the primary diploid germ cells by a peculiar type of mitosis which involves the formation of a continuous spireme and its early longitudinal splitting. The split spireme segments into four lenghts which condense to form the chromosomes. This type of mitosis is restricted to the early proliferation of oogonia and to the three divisions of each oogonium by means of which the eight cells (seven nurse cells and one egg) of each ovariole are produced. 9. Coincident with or slightly before the first appearance of haploid nuclei in the hermaphrodite gonad, degeneration of certain germ cell nuclei occurs. This continues to a variable extent throughout the development of the gonad and is particularly evident in regions where haploid and diploid nuclei meet, in diploid regions during the male phase, and in haploid regions during the diploid or female phase of the gonad. With the final establishment of the female phase degeneration usually ceases. Evidence is given that this nuclear disintegration is not directly involved in the original reduction of the spermatogonial-mother cells. 10. InIcerya purchasi collected at various places, including one collection from Italy, the same types, diploid hermaphrodites and haploid males, are present as in the American stock first described. No pure females have been found. 11. Breeding data indicate that separate strains or races differing in respect to the presence or absence of the facultative parthenogenesis by means of which the haploid males are produced, do not exist.

Notizen: Summary 1. The California race of Icerya purchasi comprises only protandric hermaphrodites and males. There exist no pure females. 2. There is never any trace of parthenogenetic development of the eggs. Each egg must be fertilized by a spermatozoon in order to initiate development. 3. The eggs are usually fertilized by spermatozoa from the hermaphrodite's own testes; that is, the hermaphrodites are self-fertilizing. 4. Self-fertilized hermaphrodites give rise to hermaphrodites only. 5. Cross-fertilized hermaphrodites, i. e., those that have copulated with males, may give rise to broods of hermaphrodites only, or to mixed broods containing a small proportion of males. 6. It is therefore suggested that only in the rare instances when a male-producing spermatozoon from a pure male succeeds in fertilizing an egg in competition with the hermaphroditically produced spermatozoa docs a pure male result. 7. The morphology and histology of the hermaphroditic gonad is described for each of the nymphal and adult instars. 8. The somatic and oogonial chromosome number in the hermaphrodites is four. The eggs undergo a normal maturation. Two tetrads are evolved and two polar bodies given off, thus reducing the chromosome number to two. No indication of the peculiar behaviour reported by Pierantoni in the maturing egg was found. 9. Many entire spermatozoa enter each egg, and evolve nuclei which show the haploid chromosome group. These nuclei develop no further unless union with the female pronuclcus takes place. Fusion of the female pronucleus with one of the male pronuclei may take place at any point between the periphery and the center of the egg. 10. In the eggs of self-fertilized hermaphrodites, i. e., those which had been isolated from males since birth, and in the eggs of cross-fertilized hermaphrodites which had copulated with males, the processes of oogenesis and fertilization arc exactly the same. There is no way to distinguish the spermatozoa of the hermaphrodite from those of the true male in fertilization. 11. Spermatogenesis is in every respect the same in the testes of male and of hermaphrodite. The somatic and spermatogonial chromosome number is four. Two tetrads are evolved and a normal reduction takes place. The cytoplasmic division is invariably suppressed during the second maturation division, and may also sometimes be absent during first division. Thus two kinds of spermatids are produced, binucleate and quadrinucleate, whose components separate upon reaching maturity. 12. Incidental observations confirm Pierantonis claim that the mycetocytes in Icerya arise from simple cleavage cells.