THE FERTILIZATION PROCESS IN THE SNAIL. 77 



of sperms pass into the female conduit and are enveloped by the 

 accessory coats of the egg along with the vitellus. 



Whether the ovarian egg actually ruptures its follicle wall or 

 merely absorbs it is not evident. However, since I have never ob- 

 served a ruptured follicle, but have occasionally found free ova 

 with a portion of the follicle wall still attached, I believe that in- 

 stead of rupturing its follicle the ovum resorbs it, as Garnault 

 ('89) describes in Hcli.v aspcrsa. Henschen ('04) describes a 

 " Zellmembran " in ovarian eggs of L. stagnalis which is probably 

 the same structure that is called a hyaline membrane in this paper, 

 but since he describes only ovarian eggs, it is impossible to tell 

 definitely whether the membrane is the same in each case. Perez 

 ('89) examined a large number of Hcli.v at varying intervals 

 after copulation but he was never able to observe ova passing 

 through the hermaphrodite duct. 



The ovum " ruptures " its follicle wall and begins the migration 

 free in company with a multitude of male germ-cells in all stages 

 of development from early spermatid to ripe spermatozoa. In 

 most instances the egg is either completely or partially enveloped 

 by a thin hyaline membrane (Figs. I, 46) which functions as a sort 

 of pseudo-vitelline membrane until it is absorbed by the ovum, or 

 disintegrates. The ovum is usually free of this membrane before 

 it reaches the hermaphrodite duct. Further evidence that this 

 membrane is related to the follicle and not to the ovum is shown 

 by the fact that occasionally free ova are found in the lumen of 

 an acinus surrounded by a membrane having one or more dis- 

 integrating nuclei adhering to it. The presence of the hyaline 

 membrane apparently bears no relation to the egg nucleus, for 

 some free eggs in which the membrane of the germinal vesicle 

 is intact have this membrane wanting entirely, while in others 

 its persists over almost the entire egg until the first maturation 

 spindle has begun to form (Figs, i, 6, 7, 12, 14 and 46). Mark 

 ('Si, p. 178) says that "the yolk is certainly not provided with 

 a distinct membrane . . . ; only a very thin shell of protoplasm oc- 

 curs in Liina.v eggs." Wierzjeski ('06) was unable to find a 

 vitelline membrane on eggs of L. stagnalis before cleavage stages 

 were reached. He states that a definite vitelline membrane has 

 been demonstrated in the eggs of the snail Paludina by Tonninges. 



