236 ROBERT CHAMBERS AND HERBERT POLLACK. 



cautions were necessary and the injections all indicated the 

 same pH as that of the external medium. 



Later, when the pluteus stage is reached the archenteron 

 occupies a considerable portion of the blastocoele. When, there- 

 fore, the pipette is inserted one is liable to injure the surface of 

 the archenteron. The resulting injury to the archenteron 

 immediately produces sufficient injury to affect the pH of the 

 fluid in the blastoccele. By avoiding injury it was found that 

 the pH of the blastocoele in the various stages of development 

 of the Arbacia until metamorphosis is identical with that of the 

 surrounding sea water. 



The extreme susceptibility to injury of the cells in the wall 

 of the archenteron is a striking phenomenon. If a pluteus is 

 injected with phenol red and the surface of its archenteron 

 slightly scratched a flash of yellow spreads over the archenteric 

 wall, quickly followed by a return to the red color of the dye in 

 the blastoccele. The cells, themselves, remain colorless and 

 apparently normal. The irritation at the spot of injury seemed 

 to spread from cell to cell and to cause them to produce an acid. 

 This resembles the production of an evanescent acid zone around 

 a mechanically injured starfish egg (5). 



The archenteric cavity of a late pluteus was also injected 

 by inserting the micropipette into the blastopore, and, in the 

 few tests performed, the pH recorded was the same as that of the 

 surrounding sea water. If too much pressure is applied the 

 blind end of the archenteron breaks and the injected fluid flows 

 into the blastoccele. 



The blastoccele of the so-called bottom swimmers were also 

 injected. These are sluggishly moving blastulae containing dis- 

 integrated material from dying cells which are squeezed out of the 

 blastular wall. The smaller size of these abnormal blastulae and 

 the accumulated debris in their interior probably account for 

 their inability to acquire the pelagic habits of the normal forms. 

 The injection of dyes into the blastoccele of bottom swimmers 

 indicate a pH below 7.0. As long as disintegration is a con- 

 tinuous process the acid reaction of the blastoccele fluid is main- 

 tained. When, however, disintegration ceases and the blastula 

 begins to recover, the pH of the blastoccele gradually rises until 

 it reaches that of the surrounding sea water. 



