36 GEORGINA B. SPOONER 



through the fluid ground substance and the persistency with 

 which the medium retains its position may be due to a physical 

 tendency to adhere to the yolk spheres or granules. 



EXPERIMENTS WITH ARBACIA PUNCTULATA 



The object of my work at Wood's Hole during the summer of 

 1909 was to carry out experiments upon the cleavage stages of 

 arbacia similar to those already described for cyclops. There were 

 also several points which had not been cleared up in the work 

 done by Dr. Morgan and myself at Wood's Hole in 1908. 



Mortality after centrifuging 



Lyon showed in 1905, that when the egg of arba'cia is centri- 

 fuged its materials are divided into four layers; an oil cap and 

 protoplasmic band in the centripetal hemisphere and in the other 

 hemisphere a broad band of yolk with the pigment massed at 

 the outer pole. He also obtained a great many normal embryos 

 from such eggs. Detailed experiments had not been made to 

 show the relation of mortality, either to the length of exposure 

 of the eggs to a high centrifugal force or to the stage in its devel- 

 opment at which the egg was centrifuged. 



Eggs and sperm from a single pair were used throughout one 

 series of experiments in order to eliminate error arising from dif- 

 ferences in animals. When six or seven lots of eggs were centri- 

 fuged in one series of experiments, the eggs and sperm used for 

 the last lot had necessarily been exposed in the bowl of sea water 

 for thirty or forty minutes, while those in the first set were fresh 

 from the animal. To meet this difficulty when the experiments 

 were repeated the sequence was changed. If in the first series 

 the first lot of eggs were exposed to the force for only a short 

 time, in the next series the first lot were exposed for a long time. 

 The counts from several series of experiments were combined. 



The speed of the centrifuge was as nearly constant throughout 

 the experiments as was possible with a hand centrifuge, varying 

 from 6000 to 7000 revolutions a minute. In making the counts 



