6 CM. CHILD 



unchanged. The swelling and separation of the micromeres 

 occur before their decoloration, but decoloration occurs in general 

 before decoloration of the macromeres. Since the cell divisions 

 do not occur simultaneously in all blastomeres and since periods 

 of increased susceptibility are associated with division periods, 

 irregularities are not infrequent, one or two macromeres some- 

 times dying before some of the micromeres, etc., but in general 

 the difference in susceptibility between micromeres and macro- 

 meres is perfectly distinct. 



In the later cleavage stages a susceptibility gradient exists in 

 the ectoderm. The swelling and separation of blastomeres 

 begins in the small cells of the apical region, progresses over the 

 ectoderm and occurs in the entodermal cells last of all. De- 

 coloration follows in general the same course. The first somato- 

 blast and mesoblast (Wilson, '92, Mead, '97 b, Child, '00), 

 which aris.e dorsally and which form respectively the ectoderm 

 and the mesoderm of the trunk region of the worm, are certainly 

 less susceptible than the cells of the apical region, and many 

 cases were observed in which the mesoblast was less susceptible 

 than the somatoblast, but further work is necessary for certainty 

 concerning this point, although other lines of evidence indicate 

 that such a difference does exist. 



Four to five hours after fertilization some of the embryos begin 

 to move, and after six to seven hours they are swimming actively. 

 At this stage (seven hours) the apical or anterior region is still 

 the most susceptible region, but a second region of high suscepti- 

 bility is arising in the basal or posterior region. The swelling 

 and separation of cells begins, first, in the apical region, but soon 

 after or at about the same time in the posterior or dorso-posterior 

 region slightly in front of the extreme posterior end, for instance 

 in the region of the somatic plate, while the intervening regions 

 of the ectoderm remain intact for a considerably longer time. In 

 one series, for example, in KNC m/200, the swelling and exten- 

 sion of cells began after two hours in KNC, usually chiefly 

 the anterior region, but often in both anterior and posterior, and 

 sometimes in the posterior earlier than in the anterior region. 

 After three hours in KNC the larvae were still moving slightly, 



