DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 5 



almost hyaline and there is a gradation in granulation basipetally 

 from it, as well as a color gradient ranging from complete de- 

 coloration apically to deep red basally. 



After three and one half hours in HgCU the swollen apical 

 region has undergone complete disintegration in many cases, 

 in most of the eggs decoloration i^ also complete, and the pro- 

 toplasm has undergone a change in appearance, being more 

 distinctly granular except where separation of granules and 

 hyaline substance has occurred in the apical region. 



The apico-basal gradient appears in these cases, both in the 

 degree of physical alteration in the protoplasm, which is greatest 

 apically and decreases basipetally, and in the progress of de- 

 coloration from the apical to the basal pole, which can be fol- 

 lowed continuously. 



With CUSO4 m/ 100000 the results are essentially the same as 

 with HgCl2 except that there is less apical swelling. In KNC 

 m/200 or m/100 after staining with neutral red, the decoloration 

 gradient is the same as in that described above, and in many 

 cases some apical swelling and disintegration occurs. 



In the living egg of Chaetopterus at the time of maturation 

 an area about the polar spindle is distinctly less opaque than 

 other parts of the egg and it is this area which is chiefly involved 

 in the swelling and protrusion and separation of hyaline proto- 

 plasm. The gradient, however, does not consist merely in the 

 difference between this area and other parts of the egg, but 

 exists in the more basal regions as well. The less opaque apical 

 area is associated with the occurrence of mitosis and the position 

 of the spindle, and the clear area about it is of course itself an 

 expression of the apico-basal gradient. 



During the earlier cleavage stages the apico-basal gradient in 

 susceptibility to the various agents appears both in the physical 

 changes of the protoplasm and in decoloration. In the eight cell 

 stage and later, the micromeres swell, become more hyaline 

 and undergo decoloration much earlier than the macromeres. 

 In many cases the egg membrane is ruptured by the internal 

 pressure and the micromeres become spherical and separate 

 completely from each other, while the macromeres are still quite 



