24 GEORGINA B. SPOONER 



of the large yolk spheres around it. All appearance of astral rays 

 is lost after centrifuging. Nothing remains but a disc of basic 

 granules, marking the center of the aster. This shows that in 

 Cyclops the astral rays are not fibers. The spindle "fibers," in 

 contrast to the astral rays, not only have the appearance of con- 

 tinuous threads but behave as such, being in some cases bent. A 

 separation of the materials composing the center of the aster 

 results from centrifuging. In the normal egg the aster is composed 

 apparently only of acid substance, but after centrifuging its 

 reaction is basic, showing that in the normal egg the basic is also 

 present together with the acid substance. 



The cleavage spindle of Arbacia may also be pushed out of its 

 normal position as a result of centrifuging. Its subsequent appear- 

 ance is in every way normal and the cleavages that follow are 

 normial. In eggs killed immediately after centrifuging the astral 

 rays extend radially into the protoplasm and are perfectly straight 

 as in the normal egg, despite the rearrangement of materials. 

 This seems to be opposed to a theory which supposes the astral 

 rays in the sea-urchin to be true fibers since it is hard to conceive 

 of a radial system of fibers remaining undistorted under such 

 conditions. 



In the remaining experiments made with Arbacia several points 

 were taken up which had not been sufficiently studied in previous 

 work. A detailed study was made of the percentage of normal 

 embryos developing from eggs centrifuged at different stages of 

 development and at various speeds. The few cases in which the 

 first cleavage after centrifuging was parallel to the layers instead 

 of perpendicular were examined. This type of cleavage occurs 

 only in Arbacia eggs which are fertilized after centrifuging. The 

 sperm pronucleus, being unaffected by the force, may approach 

 the egg pronucleus from any direction with reference to the lay- 

 ers, hence in some cases its direction is such as to cause the spindle 

 to form with its axis at right angles to the layers. The relation 

 of the first cleavage plane to the micropyle pole was studied in 

 Toxopneustes by Boveri who described it as passing always 

 through the micropyle pole. In Arbacia I found that while this 

 relation generally holds gi od there are some eggs which vary 

 widely from the rule. 



