THE pH OF ECHINODERM EMBRYOS. 235 



being returned to normal sea water, the color quickly changed 

 to that characteristic for the external medium. This procedure 

 could be repeated a number of times until there was not enough 

 appreciable color left within the blastocoele of the living embryos. 



Blastulae were also immersed in sea water whose pH had been 

 brought to 5.4 db and to 6.0 d=. Three minutes later they were 

 injected with chlor phenol red and brom cresol purple. In 

 every case the color within the blastocoele was the same as that 

 of the dye in the surrounding medium. 



Arbacia. In the Arbacia egg the hyaline plasma layer is more 

 prominently developed and much stiffer than in either the 

 Asterias or Echinarachnius egg. Consequently, the blastomeres 

 are always closely pressed together. The blastocoele appears 

 early as an enclosed cavity; it does not enlarge to the same 

 extent as in the other forms and its cellular wall, during the 

 blastula stage, is much thicker. Therefore, when a pipette is 

 introduced into the blastula a considerable amount of injury 

 usually occurs at the spot where the wall is punctured. The 

 disintegrating material of the injured cells is carried into the 

 blastocrele and the consequent addition of acidified material, 

 which has a pH of 5.4 to 5.6 (5), is sufficient to lower the pH 

 of the internal fluid to that observed by Rapkine and Prenant, 

 viz., 7.0 to 7.3. The color of the dye disappears within several 

 minutes, the time varying with the dye used and the amount 

 injected. This acid reaction in the Arbacia at this stage is so 

 readily produced that it was not suspected as an injury phe- 

 nomenon until experiments were performed on the larger and 

 less readily injured blastulae of the Asterias and Echinarachnius. 

 Extra precautions were, therefore, taken to avoid injury and 

 micropipettes were used the tips of which taper rapidly into 

 very slender and elongated shafts. With such a pipette it was 

 found possible to perform the injection with no sign of injury 

 by causing the tip of the shaft to pass between and not through 

 the cells of the blastula at the spot of puncture. The observed 

 pH was then always that of the surrounding sea water. 



When gastrulation begins the aboral end of the embryo dilates 

 and the wall of the blastocoele becomes so thin that a puncture 

 produces no appreciable injury. Consequently no special pre- 



