DIFFERENTIAL SUSCEPTIBILITY IN POLYCHETES 7 



but the ectoderm of anterior and posterior regions was largely 

 disintegrated into separate cells while the middle regions were 

 still largely intact, though giving off single cells here and there. 

 After four hours the ectoderm had completely disintegrated 

 in more than 50 per cent of the embryos, but the remainder 

 showed various stages of anterior and posterior disintegration 

 the process being usually more advanced in the anterior region. 

 A small percentage still showed slight movement at this time. 

 After five hours the ectoderm was completely or almost com- 

 pletely disintegrated in all, but the entoderm was still largely 

 intact. Decoloration in KNC or other agents, after staining 

 with neutral red, follows the same order as the swelling and 

 cell separation. 



As will appear below, the basal or posterior region of high 

 susceptibility, which is appearing at this stage, is the first physio- 

 logical indication of the growing region of the somatic plate 

 which gives rise to the ectoderm of the trunk segments. My 

 observations show that during the early development all parts 

 of the body undergo an increase in susceptibility as they become 

 physiologically younger (Child, '15 a, Chapter XV), but that 

 after the somatic plate is formed and begins to grow, its suscepti- 

 bility increases more rapidly and to a greater degree than that 

 of other parts. The first indications of this change appear at 

 the stage just described in which the susceptibility of the somatic 

 plate region is almost as high as that of the anterior region. 



During the following stages of development the relative in- 

 crease in the susceptibility of the posterior growing region con- 

 tinues. Twenty to twenty-four hours after fertilization this 

 region is the most susceptible of the whole ectoderm. The 

 swelling and separation of cells occur first of all there (figs. 6 

 and 7). Very frequently, in stages of about twenty hours, the 

 first cells to swell and protrude from the ectodermal epithelium 

 are two cells on each side of the body in the posterior region 

 (fig. 6), evidently cells of the somatic plate. Whether they 

 represent the early stages of the seta-sacs could not be determined 

 with certainty, but at this time they are the most susceptible 

 cells of the ectoderm. 



