Experimental studies on the role of ‘polar granules’ in the segregation of pole cells in Drosophila melanogaster
The early differentiation of germ cells is a common phenomenon in the animal kingdom. Insects are of special interest in this respect, as the differentiation of their primordial germ cells occurs in very early stages of cleavage (Kahle, 1908; Hegner, 1914; Reitberger, 1934; Kraczkiewicz, 1935, 1936) and the structure of the ooplasm enables relatively convenient observation of the phenomenon of germ track formation. The ooplasm is differentiated in that the posterior end of the egg contains the so-called ‘pole plasm’ in which there are easily visible inclusions quite different from yolk, though staining similarly with haematoxylin. Such inclusions are not noted in other parts of the egg. In the course of normal development the region containing granules and pole plasm always detaches, producing the primordial germ cells. During the separation of the primordial germ cells, also called pole cells, all these granules become included in their cytoplasm, and the main body of ooplasm is left devoid of them.