xii] MI AST OR, BEETLES 179 



its separation to the production of the full-grown oocytes 

 seems always to be six. 



The case of Miastor is interesting from the fact that it 

 includes a process of chromatm-diminution in the body- 

 cells, but not in the germ-cells, much as in Ascaris, but that 

 the determining cause of the development of one nucleus 

 into a germ-cell seems to be its association with the visible 

 pole-plasm at the hind end of the egg. A similar pole-plasm, 

 or a group of staining granules which corresponds with it, 

 has been observed in a number of other Diptera, and as in 

 Miastor those segmentation nuclei which come into relation 

 with it become the primitive germ-cells. Miastor is, how- 

 ever, the only insect in which it has been observed that the 

 body-cells are distinguished from the germ-cells by chro- 

 matin-diminution. 



Very similar phenomena to those found in the Diptera 

 other than Miastor have been described by HEGNER in the 

 Chrysomelid Beetles Calligrapha and Leptinotarsa. In 

 these there is a group of granules at the posterior end of 

 the egg, and when, after repeated segmentation divisions, 

 some of the nuclei come to the surface to form the blasto- 

 derm, four nuclei come into relation with these granules, and 

 instead of taking part in the general blastoderm formation, 

 they push their way through the egg-surface and form a 

 group practically outside the egg. As they do this they take 

 some of the protoplasm and the granules with them, and 

 by two successive divisions give rise to sixteen cells, which 

 are the primitive germ-cells. It appears to be a matter of 

 chance which of the segmentation nuclei come into connec- 

 tion with the granules and so give rise to germ-cells, and 

 HEGNER showed that by destroying the granules with a hot 

 needle in the fresh egg, embryos without germ-cells were 

 produced. 



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