8 C. M. CHILD 



The disintegration of the ectoderm in the posterior growing 

 region proceeds as indicated in the dorsal xievr (fig. 7) and the 

 more advanced stage of dismtegration (fig. S), in which cells 

 are also being given off from the apical region. In side view 

 (^fig. 9) the posterior area of disintegration extends around the 

 body and frequently an area of disiategration appears where the 

 stomodeum is forming, suggesting increased acti\"ity in that 

 region. 



It is of interest to note that the susceptibility of the prototroch 

 region in this stage is relatively low. a zone including the pro- 

 totroch being usually the last portion of the ectoderm to dis- 

 integrate. Apparently the cells of the prototroch have under- 

 gone a rapid decrease in susceptibility during their differentiation. 



^lesoderm and entoderm are much less susceptible than ecto- 

 derm in these stages, and the entoderm is apparently less suscep- 

 tible than the mesoderm, though after the body is covered \\*ith 

 the swoUen ectoderm cells it is often difficult to determine just 

 when the mesoderm begins to disintegrate. 



Decoloration in some killing agent after neutral red foUows 

 the same coiu-se as the swelhng and disintegration, both as regards 

 the different regions of the ectoderm, and the chfi'erences between 

 ectoderm, mesoderm and entoderm, the entoderm undergoing 

 decoloration last of all. The apparent low susceptibility of 

 entoderm is certainly not wholly due to protection by the ecto- 

 derm from the action of the killing agent, for the difference is 

 distinct while the entoblasts are stiU exposed. The entoderm 

 remains intact for hours after the ectoderm has completely 

 disintegrated and the cells are \\idely scattered. 



In the fully developed Isltysl of forty-eight hours or more, the 

 region of highest susceptibiMty is still the posterior growing 

 region and the posterior segment 'fig. 10). although in some 

 cases the susceptibility of the apical pretrochal region is almost 

 as great. At this stage cell separation and disintegration begin 

 in the growing region, just in front of the extreme posterior tip, 

 and progress anteriorly over the segments, but much more 

 rapidly ventraUy than dorsally (fig. 11), that is, the ventro- 

 latero-dorsal gradient characteristic of the bilateral invertebrates 



