GERM-CELL. ORIGIN IN INVERTEBRATES.
The clearest and most striking case of early germ-cell differentiation among the invertebrates is that of Ascaris megalocephala, in which Boveri (1892) traced the germ-cells through the cleavage back to the 2-cell stage. The conspicuous differential feature of this cell at the 2-cell stage (and to the sixth cleavage) is the maintenance at mitosis on the part of one of its daughter-cells of the integrity of the cluvmo somes in contrast with the soma-cell, both of whose daughter-cells suffer a diminution of the chromatin at each division. The germ-cells become segregated at the 32-cell stage of cleavage, after which they produce only similar germ-cells at each division.
Previously Balbiani (1885) had succeeded in tracing the germ-cells (" pole-cells") of the dipter Chironomus to an early stage of differen tiation in the segmenting egg. "Pole-cells" were first reported by Robin in 1862 for certain other Diptera, and subsequently (1863) also by Weisman for Chironomus. Hasper has quite recently (1911)
identified one of the first four cleavage-cells as the progenitor of all the germ-cells in Chironomus.
A similar history was traced by Hacker for the germ-cells in Cyclops; here the germ-cell progenitor is recognizable at the 32-cell stage.
Hegner (1909, 1914, 1915) has worked out very completely the early germ-cell path ("Keimbahn") in certain Diptera and in chrysomelid beetles, where the diagnostic marks are a polar differential granular cytoplasmic content, the "pole-disk," constituting a germ-cell determi nant. In the dipter Minaslor, Hegner (1914) describes a segregation of the germ-cells at the 8-cell stage, in a number of other cases at the 32-cell stage.
The literature touching the germinal path ("Keimbahn") in inverte brates is fully and very ably reviewed by Hegner in the Journal of Morphology (vol. 20, p. 231; vol. 25, p. 375; and vol. 26, p. 495). The reader is referred to these papers for bibliographic lists.