LARVAL DEVELOPMENT IN THE SEA URCHIN 103 



Acids 



Development from the first cleavage in HC1 m/1000 proceeds 

 slowly, reaching the blastula stage in about 24 hours; with 50 

 per cent more or less of deaths, mostly total. During the next 

 two or three days the mesenchyme cells enter the blastocoel, 

 and after four or five days, gastrulation may begin. Even in this 

 concentration the gastrulae begin to show apical outgrowths 

 like figures 25, after six days in the solution, but do not usually 

 develop further, and even if returned to water at this time they 

 die in a day or two. 



In HC1 m/2000 from the first cleavage, a few total deaths 

 occur and more or less of the apical region dies in 20 to 40 per 

 cent at or before the blastula stage. From this stage on, dif- 

 ferential acclimation begins, and the whole gastrulae are like 

 figure 25, the preplutei, like figures 27 and 28, and the final 

 stages range from the extreme types of differential acclima- 

 tion shown in figures 31 to 36, through skeletal forms with 

 apical outgrowth representing the oral lobe and with or with- 

 out basal ciliated band, like -figures 37, 60, 61, 64, and rounded 

 forms with small apical outgrowth like figures 49, 51 to the 

 lower limits of differential acclimation like figures 50, 52, 53, 

 78 to 81. In fact, practically every individual which lives through 

 the gastrula stage, even as a partial basal form, shows at least 

 some degree of differential acclimation. 



In HC1 ra/5000 from the first cleavage there are usually no 

 deaths, and apical outgrowth begins in the gastrula (fig. 25), 

 and the resulting forms range from wide-angled plutei with 

 large oral lobe (figs. 23, 24, 29 to 32), through normal plutei, 

 to inhibited forms with apical acclimation (figs. 37, 60, 61, 64, 

 49, 51). The extreme types of differential acclimation like 

 figures 33 to 36 do not usually appear in this concentration. 



In HC1 ml 10000 from the first cleavage, all develop into wide 

 angled plutei with large oral lobe, about 50 per cent approach- 

 ing normal, like figures 23 and 24, and the remainder ranging 

 from these to forms like figures 29 to 32. 



In HC1 m/50000 from the first cleavage, 50 per cent, more or 

 less, are normal, the remainder slightly wide angled with large 



