12 



BIOLOGICAL LECTURES. 



sense this is undoubtedly true. But it is difficult to believe that 

 all of the material of the rays comes from the base {i.e., from 

 the nucleus or the centrosome), for they often extend themselves 

 throughout the entire cytoplasm, even in cases where, as in the 



^''■' :'^H.../i■••4>.<.l...#•<"%•■■f■•l.,..4'■•^•: 



a 





r>-i 



fc 



>?^ 



v.? C-i-S" 'i-v-- 





Fig. 3. — (a) Protoplasm and yolk-spheres from the egg of Tlialassevia in section. The upper 

 part of the section shows the result of prolonged extraction of the dye (iron-haematoxylin), 

 the lower half represents varying degrees of extraction (1200 diameters); (b) egg of Nereis 

 in section showing yolk-spheres and the first polar amphiaster above (600 diameters). 



sperm-aster of echinoderms, the centre of the aster remains very 

 small, and the nucleus still consists of a compact mass of chro- 

 matin (Fig. 4). It is more probable that they grow at the tip, 

 continually extending themselves at the cost of the material 

 lying in the meshwork. When the rays are followed out periph- 



