SECT. 5] 



IN ONTOGENESIS 



811 



•StrongylocentroUiS whole period 

 □ • » between 4 c 



stage S^blastula (primitive 



mesenchyme) 

 B ■Strongyjocentrotus between blastula&, 

 3 spicule stage 



Strongylocentrotus &.Echinu8 p® 



•9 2-0 



2-2 2-3 2-4 2-5 2'G 



38 -40 '42 -44 -46 -48 -50 -52 '54 -56 '58 -60 -62 

 Rana • 



in Fig. 195. In the case of the frog, the curve appears to drop only 

 on one side. The zone within which normal segmentation would 

 proceed also differed markedly for the two echinoderms; for in- 

 stance, it was A — 0-63° in the 

 case oi Echinus and only —0-57° 

 in the case of Strongylocentrotus, 

 while in the case of Rana it was 

 only — 0-17°. On the other 

 hand, the zone between the os- 

 motic pressures which caused 

 death was relatively wider in the 

 case of the amphibian than in 

 the case of the two echinoderms. 

 The effect of osmotic pressure 

 on the embryos, moreover, was 

 not the same at all stages. As 

 regards mortality, the sensitivity 

 of frog embryos to hypertonic 

 solutions augmented with age; 

 thus at the 8-cell .stage they were 

 killed in 24 hours by solutions of 

 A -^.0-95°, but at the time of hatching they we^ killed in 24 hours 

 by a solution of only — 0-43°. On the other hand, the embryos of 

 Strongylocentrotus lividus supported changes of osmotic pressure better 

 the^older they were. As regards speed of development between given 

 stages, this also was affected differently. As is seen from Fig. 195, 

 between the 4-cell stage and the appearance of the first mesenchyme 

 cells, the speed of development falls off distinctly less slowly from 

 the optimum with change of osmotic pressure than it does between 

 the appearance of the first mesenchyme cells and the three spicules 

 of the pluteus. 



Faure-Fremiet made parallel experiments on the eggs of Sabellaria 

 alveolata, and obtained precisely similar results. Fig. 196 taken from 

 his paper shows the usual bell-shaped curves, resulting from the plot 

 of active osmotic concentration against the time required to reach 

 certain stages of development. There is a certain optimum os- 

 motic pressure, from which the speed of development rapidly falls 

 away on both sides; thus, although the processes of division will 

 go on between the limits of 0-85 and 1-48 mol. per litre, the 



Fig. 195- 



