THE FERTILIZATION PROCESS IX THE SNAIL. 87 



with few exceptions they are tangent at the first (Fig-. 37), they 

 soon become somewhat appressed as in Fig. 29. As the sperm 

 pronucleus comes in contact with the egg nucleus, the karyomeres 

 of each nucleus come to lie closer and closer together so that by 

 the time the nuclei have become appressed the number of chro- 

 mosome vesicles in each has been reduced by fusion. This fu- 

 sion continues so that the identity of each pronucleus is lost (Figs. 

 35 39)- Ultimately all the egg chromosome vesicles fuse to form 

 a typical pronucleus surrounded by a distinct and apparently con- 

 tinuous membrane. The definitive sperm pronucleus is formed 

 at the same time and in the same manner (Figs. 39, 41). Chro- 

 momeres are abundant in both pronuclei but appear to be separate 

 rather than " strung on a limn thread " in resting nuclei as well as 

 in fusing pronuclei (Figs. 41-43). Thus the formation of the 

 pronuclei and of the first cleavage nucleus in L. s. appressa is es- 

 sentially parallel with that in the worms, Eust\lochiis (Planarian), 

 (Van Name, '99), Dinophilus (Nachtsheim, '19) ,Chcetoptcnis, 

 (Mead, '95), Nereis (Lillie, '12), Platynereis (Just, '15), and 

 Amphitrite (Scott, '06) ; an echiuroid, Thalassema (Griffin, '99) ; 

 the hymenoptera Acrochisrnus and the bug Icerya (S. H. Schrader, 

 '24, '25) ; the mite TetranycJius (F. Schrader, '23) ; the daphnid 

 Polyphemus (Buchner, '15, citing Kuhn) ; the nudibranchs Doris 

 and Montagua (Smallwood '05), and other forms. 



Kostanecki and Wierzejski ('96) figure chromosome vesicles 

 which appear as lobules on both pronuclei (Taf. XX., Figs. 25, 

 28). The absence of definite chromomeres in their pronuclei 

 possibly indicates that little attention was given to staining and 

 studying the nuclear substances. The fusing pronuclei lose this 

 lobulated appearance and have definite nuclear membranes (Taf. 

 XX., Figs. 30-34) as is the case in those forms which are known 

 to form the egg pronucleus from karyomeres. Mark's figures 

 ('81) suggest a vesiculated stage in the pronuclei of Liina.v 

 campestris, but he does not recognize such a condition in his text 

 description. Griffin ('99) shows a distinctly vesiculated egg 

 nucleus in Thalassema (Fig. 29) and vesiculated individual chro- 

 mosomes in the telophase of the second maturation division (Fig. 

 55)- 



