xn] MIASTOR 177 



other. They suppose that this division is a differential 

 one, and that one of the cells formed by it gives rise 

 to both the testes, the other to the ovaries. Here again 

 BUCHNER disagrees, and maintains that the separation of 

 the primitive spermatogonium from the primitive oogonium 

 takes place at the next division, when the two primitive 

 germ-cells divide into four, but on the whole Miss STEVENS' 

 evidence points more strongly to the former opinion. In 

 Sagitta, therefore, as in Ascaris, a single primitive germ- 

 cell is differentiated at an early stage of segmentation by 

 the presence of a body that is absent from the other cells, 

 but in Sagitta the body is of unknown origin and not cer- 

 tainly chromatinic, while in Ascaris it consists in parts of 

 the chromosomes. 



A third case of the very early differentiation of a primitive 

 germ-cell is provided by the paedogenetic eggs of Miastor, 

 a fly of the Dipteran family Cecidomyidae. The main out- 

 lines of the process were observed more than fifty years ago 

 by LEUCKART and METSCHNIKOFF, and more recently by 

 KAHLE (1908) in Europe, and HEGNER (1914) in America. 

 Miastor is remarkable in having larvae which reproduce 

 viviparously by a process of parthenogenesis ("paedo- 

 genesis"). The mature larvae have two ovaries, each con- 

 taining typically thirty-two oocytes, though the number 

 may be smaller owing to the suppression of some of them. 

 The oocytes are peculiar in having attached to them a group 

 of nurse-cells that are not derived, as in most other insects 

 from oogonia, but are immigrant mesoderm cells. At the end 

 of the egg opposite the nurse-cells is a patch of protoplasm 

 called by HEGNER the pole-plasm. The egg-nucleus under- 

 goes a single, non-reducing polar division, and the mature 

 nucleus then begins the segmentation mitoses in the central 

 yolky portion of the egg. When two divisions have taken 



D. C. 12 



