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SCJKNCE PROGRESS 



The last-named problem has recently been solved by the 

 writer (5). The "nucleolo" is not a nucleolus which has 

 escaped from the germinal vesicle, but consists of chromatin. 

 Because of its constitution and fate I have called it keimbahn- 

 chromatin. All of the chromatin in certain young oocytes 

 becomes arranged as double rods on an asterless spindle. This 

 spindle does not proceed to separate the chromosomes, but 

 gradually condenses and shortens until the chromatin becomes 

 a homogeneous mass in the form of a Greek cross. This cross 

 later becomes approximately spherical, and usually occupies 

 a position near the posterior end of the fully developed oocyte. 

 Oocytes which possess keimbahn-chromatin are without a 

 germinal vesicle. How the egg is provided with a nucleus 

 appears to be as follows : Two oocytes which chance to lie 

 end to end in an ovarian tubule develop differently probably 

 because of their relative positions ; the spindle of the one 

 posteriorly situated never completes its normal function, but 

 disappears as the chromosomes condense to form the keimbahn- 

 chromatin. The chromosomes on the spindle of the anterior 

 member of the pair disintegrate and give rise to a nucleus. 

 The two oocytes then fuse end to end ; one (posterior) furnishing 

 the keimbahn-chromatin, the other (anterior) providing the 

 nucleus. A number of stages were found in my material which 

 indicate that such a process may occur. If further investigation 

 proves this to be true, then every egg laid by these hymenop- 

 terous parasites consists of two oocytes united end to end. 



The development of the psedogenetic larvae of Miastor 

 furnishes us with the best example of the keimbahn in any 

 animal. Kahle's(6) careful work on Miastor metraloas has been 

 confirmed by myself (7) for M. amcricana. In the eggs of these 

 species the keimbahn-determinants are present in the form 

 of a deeply staining mass of substance (the " polares plasma " 

 of Kahle) located at the posterior end, which becomes visible 

 shortly before the maturation processes are initiated. One of 

 the first eight cleavage nuclei fuses with the polar-plasm, and 

 with it is cut off from the rest of the egg. This is the first 

 cell formed, and is the primordial germ cell. It differs from 

 all of the somatic cells in two important respects : (1) it contains 

 all of the polar-plasm, and (2) its nucleus is provided with a 

 complete amount of chromatin, whereas the nuclei of all of 

 the somatic cells lose part of their chromatin during two 



