No. 6.] THE EGG OF UXIO. 271 



believe that the inner sphere has necessarily any such definite 

 morphological value as this would seem to imply. For it may 

 disappear between the first and second maturation divisions, 

 and is then reformed, as the first step in the prophase of the 

 second maturation spindle, from the cytoplasm. The same 

 method of formation may also be observed in other places (e.g., 

 formation of the first cleavage spindle). 



Both Van Beneden's and Boveri's conceptions of the struc- 

 ture of the aster appear as phases in the history of the mitosis, 

 though Boveri's " centrosome " is really the inner sphere, and 

 his " Centralkorn " or " centriole " really the centrosome. 



In conclusion, I shall sum up the evidence which the study 

 of the egg of Unio has furnished against the theory of the 

 permanency of the centrosome as a unique organ of the cell, 

 combining an earlier paper 1 with this. 



1. A sperm amphiaster is formed, but it disappears utterly 

 at the time of the metaphase of the first maturation spindle. 



2. Entirely independently of the sperm and egg asters, there 

 arises in the egg of Unio at the time of the metaphase of the 

 second maturation spindle an accessory aster, in the center of 

 which is a minute centrosome. This centrosome divides and 

 a small amphiaster is formed, which entirely disappears at the 

 beginning of the telophase. 



3. After the formation of the second polar globule the egg 

 centrosome disappears. 



4. The two cleavage centrosomes arise independently of any 

 of their predecessors, and apparently separately. 



5. Fission products of the centrosome become cytomicro- 

 somes. 



Thus the egg of Unio furnishes evidence : in the first place, 

 that the centrosomes are not necessarily genetically continuous ; 

 in the second place, that a centrosome may arise in the general 

 cytoplasm (accessory aster) ; and, in the third place, that 

 products of the centrosomes may become other formed ele- 

 ments of the cell. If this last centrosome phenomenon should 



1 Lillie, Frank R., "On the Origin of the Centers of the First Cleavage 

 Spindle in Unio." See report of the meeting of the American Morphologists in 

 Boston in Science, vol. v, p. 114. March, 1897. 



