74 



JOHN W. MACARTHUR. 



dilute neutral red since the stained tissue decolorizes sharply at 

 the death point. Since general susceptibility increases steadily 

 from fertilization to gastrulation the concentration of the agents 

 was correspondingly decreased in the later stages. 



The order of susceptibility in the succeeding stages of develop- 

 ment is shown in Fig. 10. Disorganization, manifested by droplet 



a 





o 







f 



FIG. 10. Disintegration of normal sand-dollar eggs and embryos, showing re- 

 lative susceptibility of parts. Mesenchymc omitted, a-b, eggs; c, 4-celled; d-f, 

 blastulie; g-j, gastrulte and early larvae. When the polar axis becomes recognizable 

 disintegration proceeds from animal to vegetal pole (i.e. is apico-basal) . 



formation, cell separation, roughening of contour, visible dis- 

 integration, and decolorization, according to the particular agent 

 used, begins at one point of the surface of the egg, or at one pole 

 of the blastomeres in tin- 2- or 4-cell stage, or at one pole of the 

 cleavage mass or early blast ula, and then extends gradually over 

 adjacent parts to reach last the opposite surface or pole. 



As soon as the blast ul.i differentiates to show the b.is.il thick- 

 ening and begins its elongation, one may orient the embryo in 



