LINVILLE: PULMONATE GASTEROPODS. 245 



cell, the disappearance of the egg-centrosome and the egg-aster, though 

 long delayed, is complete. 



A reduction division of the chromosomes in the Roux-Weismann sense 

 was observed in the second maturation division of the egg of Limnaea 

 elodes. The process consisted of a transverse division of the dyads re- 

 sulting from the longitudinal splitting of the masked tetrads of the first 

 maturation division. 



In fertilization the tail follows the sperm-head into the egg, but later 

 is resorbed by the egg cytoplasm. The sperm-head, after breaking away 

 from the tail begins, under the influence of the sperm-centrosome, to 

 move, with its base in advance, toward the egg-aster. At the beginning 

 of this movement the sperm-centrosome is not visible ; later it becomes 

 distinctly visible, and often very large, giving evidence of variations 

 comparable in a measure to the variation in the egg-centrosome of the 

 first maturation spindle of Limnaea. 



Occasionally the sperm-centrosome of Limnaea divides into two while 

 on its way toward the egg-nucleus, a process prophetic of the existence 

 of these centrosomes at either pole of the first cleavage spindle. 



On account of the fact that in Limnaea both egg-centrosome and 

 sperm-centrosome at certain stages disappear for a time, it is impossible 

 to ascertain the source of the first cleavage spindle by following t .e 

 history of the centrosomes. However, reasonably satisfactory proof 

 that the first cleavage spindle is wholly of spermatic origin is found in 

 the facts that the incipient cleavage spindle involves the sperm-nucleus 

 first, and that the egg-nucleus, so far as my observations have extended, 

 is never involved in the spindle before the sperm-nucleus is entirely 

 drawn upon it. 



