THE FERTILIZATION PROCESS IN THE SNAIL. 83 



of ovarian insemination is shown in Fig. u, in which an ovum 

 that has not yet reached the hermaphrodite duct proper contains 

 four sperm heads and a body that is probably a degenerating or 

 abortive sperm pronucleus. The location of 'this ovum and the 

 presence of a hyaline membrane over a considerable area of its 

 surface indicate that it has only recently left its follicle, while the 

 forming archiamphiaster and the round-head sperms argue for 

 an earlier insemination than would normally occur in an egg in 

 this region of the ovotestis. Fig. 14 shows another ovum which 

 similarly indicates ovarian insemination. 



The presence of spermatozoa in ovarian ova has been described 

 by Buchner ('14, '15) in the Archiannelid Saccocirrus and in one 

 of the Turbellaria, and by Nachtsheim ('19) in Dinopliilus tiputris. 

 Lams ('07) found in the slug Arion empiricorum that " fecunda- 

 tion was often intraovarine " and that segmentation stages, in- 

 cluding the morula and blastula, occur in the interior of the ovo- 

 testis. " The mass of blastomeres was entirely surrounded by 

 follicle cells. One is able therefore to say that in Arion the de- 

 velopment sometimes begins in the ovary." 



C. Normal Insemination. 



From my material of L. s. apprcssa it is evident that the earliest 

 insemination normally takes place within the acinus in which the 

 ovum and the spermatozoa which enter the ovum are developed. 

 Additional sperms continue to enter the ovum until it passes out 

 of the hermaphrodite duct, but it is clear that one and only one 

 sperm pronucleus fuses with the egg pronucleus. The super- 

 numerary sperms normally disintegrate, or possibly pass through 

 the ovum before the egg pronucleus is formed (Figs. 14, 32). 



Since the yolk granules interfered with studying the sperms, 

 several living adult L. s. appressa were centrifuged and im- 

 mediately fixed in a warm fixing fluid. In one of the snails there 

 were five free ova in the acini. In two of these which had very 

 recently lost their hyaline membranes, marked cytoplasmic dis- 

 turbances resulted from centrifuging them. The periphery of 

 each ovum suffered severe fragmentation and dissociation of the 

 yolk granules. Although much cytoplasm remained around the 

 germinal vesicle the yolk granules were sufficiently displaced to 



