5 GAMETES 277 



have been observed in the dark iield microscope (Runnstrom, 1928/29). 



The contents of the egg are liquid. They can be stratified by cen- 

 trifugation into layers of yolk, fibrillar cytoplasm and enchylema with 

 mitochondria (Fig. 113, p. 195). The cytoplasmic fibrils are double 

 refracting; they carry the ribonucleic chromidia (Monne, 1946a). 



The egg of Tubifex has a much thinner cortical layer, which is easily 

 destroyed by lipid solvents. The ground cytoplasm consists of fibrils 

 which are beaded by chromidia. In the electron microscope the 

 chromidia measure 0.15 /^ (Lehmann and Biss, 1949). The fibrils, 

 whose diameter is smaller than o. i fx, form a coarse meshwork which 

 harbours the microscopic yolk granules (ca, 2 fi diameter). This gel 

 can loosen its junctions, so that the fibrils display protoplasmic flow, 

 which is the case during anaphase and telophase of mitosis. In this 

 state the egg content is liquid and can be stratified by centrifugation 

 in the same way as described for the sea-urchin egg. 



At the two poles the protoplasm of the Tubifex egg is clearly 

 differentiated into regions of animal and of vegetal cytoplasm. These 

 differentiations are microscopically visible because they contain 

 strongly basophilic granules ^Fig- 158)- Prior to fertilization the cyto- 

 plasm of either pole can be forced across the cell by centrifugation 

 and united with the cytoplasmic opposite pole (Lehmann, 1948). There 

 is no mixture with the central fibrillar cytoplasm. The stratification 

 produced is stable in Tubifex eggs, whereas in other cases, as e.g. in 

 Limnaea eggs {Molhisca), the original arrangement is restored by 

 protoplasmic flow (Raven and Bretschneider, 1942). 



Tubifex eggs with displaced polar cytoplasm can develop normal 

 embryos. But they do not do so when the centrifugation has been 

 applied too early. Lehmann (1948) thinks that the polar regions 

 differentiate at the expense of the yolk and that their development is 

 interrupted after the centrifugal displacement. Since, during the 

 cleavage of the egg, the polar cytoplasm can be traced into definite 

 somatic cells, it can be shown that in germs with abnormal develop- 

 ment those somatic cells contain too small a portion of polar cyto- 

 plasm. This shows how local regions of the egg are capable of inducing 

 the development of definite parts of the germ. For that reason, there 

 are not only multicellular organizers which control the organo- 

 genesis during the development of the embryo, but there are already 

 regulating systems on a lower scale inside the egg cell. In this way 



