DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 11 



more fluid, for a stratification of the cell contents with reference 

 to gravity appears, particularly in the micromeres of the earlier 

 cleavage stages. Such a stratification occurs in Chaetopterus, 

 but is more distinct in Nereis stained with neutral red, because 

 of the greater transparency of the cells. When the four-cell 

 stage is killed in the proper concentrations, the higher suscepti- 

 bility of the apical region of each cell, which would later give rise 

 to a micromere, is indicated by the appearance 'of stratification 

 there, while other regions are unchanged. This protoplasmic 

 stratification precedes death, but shows the same regional differ- 

 ences as regards susceptibility. As the cells become smaller with 

 advancing development, these changes become less conspicuous. 

 The relation between physiological condition of the cells and 

 their ability to hold neutral red can be demonstrated very clearly 

 in Nereis. In the earlier cleavage stages, where both micromeres 

 and macromeres are directly exposed to the neutral red, both 

 stain very readily and deeply and undergo decoloration only 

 as they die. In later stages, for example, in young, nearly 

 spherical, swimming trochophores, in which the ectoderm has 

 completely overgrown the entoderm, the ectoderm stains rapidly 

 and deeply, but little or none of the neutral red reaches the 

 enclosed macromeres until the ectoderm cells approach death. 

 Then they are unable to hold the neutral red and it passes 

 through them to the macromeres which stain rapidly. The 

 decreasing ability of the ectoderm cells to hold neutral red as 

 they approach death can be shown even more clearly in another 

 way. Young trochophores are placed in KNC, HgCl2 or some 

 other agent used and after various periods of exposure to the 

 killing agents are removed to a dilute solution of neutral red in 

 sea water. After short periods of exposure to the killing agent 

 the ectoderm stains rapidly and the macromeres very slowly or 

 not at all, as in normal forms. Later as the ectoderm cells 

 approach death they stain much less rapidly and less deeply, 

 but the enclosed macromeres now stain more rapidly. After 

 still longer periods of exposure the ectoderm is dead and does not 

 stain at all but the entoderm still stains very rapidly and deeply. 

 Since, in order to reach the entoderm at this stage, the neutral 

 red must pass through the ectoderm, and since the macromeres 



