6 EDWARD DRANE CRABB. 



occurred with them. Figs. 20-25 show several stages in the 

 formation of the sperm amphiaster in virgin eggs. Fig. 25 repre- 

 sents an early stage in the formation of the vesiculated sperm pro- 

 nucleus and is the only one of its kind found. 



Kostanecki and Wierzejski ('96) found that in eggs of Physa 

 fontinalis which had been reared in mass cultures the sperm 

 amphiaster is often formed before the anaphase of the first 

 maturation division (Taf. XVIII. , 3, 4) and that the sperm pro- 

 nucleus may have formed before the second polar body is given 

 off (ibid., 4). Byrnes ('99, p. 215) states that in Lima.v agrcstis 

 the " relations of the sperm head, the asters and the maturation 

 spindle are precisely similar to those figured by Kostanecki and 

 Wierzejski for Physa." Thus the work of Kostanecki and 

 Wierzejski on Physa eggs, that of Byrnes on Liwiax eggs as well 

 as my work on virgin L. s. appressa eggs shows that there is no 

 real relation between the maturation activities of the egg and any 

 stage in the development of the sperm amphiaster or pronucleus. 

 Rather, it appears that the state of the development of the sperm 

 pronucleus is related to the time at which the selected sperm 

 entered the ovum. The work of these investigators indicates that 

 the process of fertilization in their " normal " Lima.v and Physa 

 eggs is quite similar to that in my " virgin " eggs. 



F. Formation and Fusion of the Pronuclei. 



Male Pronucleus. After the head of the spermatozoon has 

 reached its maximum size it loses its ability to take basic stains and 

 appears as a hyaline area or vesicle in the egg cytoplasm (Figs. 

 19, 21, 25). Although there appears to be no connection between 

 them, the sperm amphiaster is associated with the vesiculated head. 

 Kostanecki and Wierzejski ('96) show several figures of Physa 

 eggs in which the amphiaster has migrated a relatively great dis- 

 tance from the sperm head before the latter formed a pronucleus. 

 In my material the sperm pronucleus becomes vesiculated about 

 the time the second polocyte and the egg pronucleus are formed 

 of definite karyomeres. 



Female Pronucleus. Following the telophase of the second 

 maturation division, the egg chromosomes form vesicles which 

 appear to be more or less independent of each other. Although 



