276 Robert Willielm Hegner. 



psedogenftic larva of Miastor contains twelve to fifteen nnclei, "Man 

 bcmerkt znniichst, class der am spitzen Pole des Pseudovums liegende 

 Keimkern von einer dicken dunkeln Dottermasse scharfer umgeben 

 wird und niit dieser znsammen bald in eine besondere, 0.017 mm. 

 grosse, membranlose Zelle sich abschniirt." This gives rise to the 

 pole-cells. Schwangart (1905) fonnd that the primitive germ-cells 

 of Endromis "sind grosser und dotterreicher" than the blastoderm- 

 cells. 



The theory that the yolk contained in the germ-cells is transformed 

 by them into the energy of motion is strengthened by the fact that 

 the migrating germ-cells of many species of vertebrates also contain 

 yolk-globules (Beard, 1902, in Elasmobranchs ; Eigenman, 1892, in 

 Teleosts; Nnssbanm, 1880, in Amphibia; Allen, 1906, in Reptilia). 



2. The Migration of the Priinitive Germ-Cells in the Insecta. 

 A. The Migration of the Pole-Cells through the Pole-Cell Canal. 



The authors who first described the pole-cells of insects (Robin, 

 1862; Weismann, 1863) supposed that they took part in the forma- 

 tion of the blastoderm. This interpretation was corrected by Metsch- 

 nikoff (1865) and Leuckart (1865) who maintained that the pole- 

 cells develop into the germ-gland. These two authors, however, did 

 not tell us how the primitive germ-cells got back into the egg after 

 their complete separation at the posterior end. Balbiani (1882-5) 

 added the evidence necessary to prove that the pole-cells really become 

 the reproductive organs, but he was unable to determine whether 

 they force their way through the blastoderm, or pass into the egg 

 by way of a clear space left for their entrance. Sections of the 

 eggs of Cliironomiis led Ritter .(1890) to believe that the pole-cells 

 move of their own accord through a gap in the blastoderm which then 

 closes after them. 



Escherich (1900) in Musca made the first accurate study of the 

 passage of the pole-cells from the posterior amniotic cavity into the 

 embryo. Many of his figures show conclusively not only '^die Wan- 

 derung der Polzellen durch die Iveimplatte" but also an "intercellular 



