THE pll OF ECHINODERM EMBRYOS. 



237 



II. IMMERSION EXPERIMENTS. 



Embryos in all stages of the starfish, sanddollar, and sea 

 urchin were immersed in bowls of normal sea water deeply 

 colored with brom cresol purple, phenol red, meta cresol purple 

 and cresol red. After an immersion of ten minutes to half an 

 hour the embryos were removed, rapidly washed, and placed in 

 fresh sea water. In every case, the dye penetrated the blastoccele 

 and indicated a pH of the seawater. These colored embryos were 

 then placed in sea water having a pH of 6.6 db (sea water acidi- 

 fied with KH 2 PO 4 ) and the color changed, within a few seconds, 

 to that characteristic of the environing sea water. 



III. CONCLUSION. 

 The microinjection of dye indicators (see Table), and the 



immersion of embryos in sea water colored with dye indicators 

 show that the fluid in the blastoccele of the normally developing 

 embryos of Asterias forbesii, Echinaradmius parma, and Arbacia 

 punctulata has the same pH as that of the environing sea water. 

 This is true for all the stages of the embryos from the time that 

 the blastocoele first appears until metamorphosis. 



The fact that the color of the dye in the blastoccele always 

 changes within a few seconds to that typical for the pH of the 

 environing medium whether acid or alkaline, and the fact that 

 the color disappears from the blastocrele of embryos in sea water 

 in which no dye was present, indicate that the wall of the 

 blastoccele of a normal embryo is freely permeable. 



The variations in the blastoccelic pH recorded by previous 

 investigators can be accounted for by the ease with which acid is 

 evolved in the sea urchin embryo when the cells of the walls of 

 the blastula and the archenteron of the pluteus are injured. 



17 



