3. The maturation and fertilization of the ova. 27 



3. The maturation and fertilization of the oya. 



(Plate 3 figs. 1—22.). 



The processes of maturation which take place immediately before and for some time 

 after the extrusion of the ova into the mantle-cavity, cannot be followed in the living ova, 

 partly because of the obscurity caused by the yolk, and partly because the eggs apparently 

 will not develop when taken out of the mantle-cavity. My observations therefore are all 

 made on fixed and stained material (fixation in corrosive and acetic, staining with borax-carmine, 

 iron-haematoxylin etc.). There is however little or no danger of error in following the series 

 of changes because the eggs of one and the same brood are often at a slightly different stage 

 of maturation, though substantially the same; and by observing the eggs in the act of being 

 extruded into the mantle-cavity and then preserving the various batches at different periods 

 after this extrusion, a chronological series is obtained. 



The chronological order of events in Peltogaster curvatus is roughly as follows, but much 

 individual variation apparently pertains. 



Stage Hours after extrusion into mantle-cavity. 



1 st polar spindle to formation of 1 st polar body I — 3 hours 



2 nd polar spindle and formation of 2 ud polar body 2 — 5 hours 



Fertilization and migration of pronuclei 3 — 7 hours. 



We will now describe in detail these processes in Peltogaster curvatus. 



The first maturation spindle of the ovum (sp x ) is formed while the latter is still in the 

 ovary, lying amid the germinal epithelium and immature ova at various stages of growth 

 Plate 3 fig. 1). The spindle is at first directed with its long axis parallel to the surface of 

 the egg. The chromosomes which are at first irregularly scattered along the spindle, show 

 signs of the typical tetrad formation as exhibited in Plate 3 fig. 2, but when they are arranged 

 in the equatorial plate the tetrad arrangement is no longer visible and it is possible to count 

 between twelve and fourteen dumb-bell shaped chromosomes (figs. 3 and 3a). At about the 

 period when the spindle begins to rotate the maturing eggs are expelled into the mantle- 

 cavity, passing through the colleteric glands on either side of the body. As soon as the eggs 

 have passed into the mantle-cavity the spindle is found to have rotated so that its long axis 

 is at right angles to the egg-surface (figs. 4 and 4 a). In figures "> and 5 a Plate '.1 a trans- 

 verse section is shown through the spindle. 



The spindle now passes into telophase and the first polar body is extruded, the second 

 polar spindle being immediately formed without any reconstitution of the nucleus (fig. 6). 

 A view of the equatorial plate of the second maturation spindle is given in the drawing of 

 the whole preparation (fig. 7). Fourteen chromosomes are visible. 



4* 



