The maturation and fertilization of the egg of Cerebratulus. 441^ 



As the outer pole of the spindle approaches the periphery , the 

 surface of the egg is slightly depressed in its vicinity. This depression 

 soon disappears, and a corresponding elevation takes its place. In 

 this elevation lies the outer pole of the spindle with its two minute 

 centrosomes pressed close to its outer border (Fig. 9). At this stage 

 the centrosphere of the outer pole has nearly disappeared, and the 

 centrosomes lie practically free in the general cytoplasm (Fig. 15, 17). 

 After attaining considerable prominence the projection becomes slightly 

 constricted near the surface of the egg in preparation for its final 

 separation as the first polar body. Before it is entirely separated 

 from the egg-cell its two minute centrosomes separate more widely 

 at right angles to the axis of the polar spindle. This separation is 

 sometimes carried so far that the whole outer portion of the spindle 

 becomes divided, and the spindle-fibres have two foci on the outer 

 pole instead of one. Such a spindle is shown in Fig. 16, where the 

 inner centrosomes are also separated. With spindles like these the 

 centrosomes of the outer pole pass to opposite sides of the polar body 

 even before this latter has been entirely cut ofi (Fig. 19). Long 

 before the polar body is separated the last traces of aster-fibres about 

 its centrosomes have disappeared, but some of the spindle-fibres may 

 be seen up to the time of actual separation (Fig. 20). As the divided 

 chromosomes pass towards the poles, the spindle-fibres become more 

 and more irregular and indistinct (Figs. 18 — 20). At the moment 

 when the polar body is cut oti' the few spindle-fibres remaining are 

 pressed closely together, and thicken up very slightly at their middle, 

 but they immediately disappear, and no perceptible Zwischen - 

 kör per is formed. 



So much for the achromatic structures. Let us now briefly con- 

 sider the chromosomes. It was noticed above that a very minute 

 portion of the chromatin of the germinal vesicle formed itself into 

 sixteen somewhat irregular but, on the whole, more or less spherical 

 or ring-shaped chromosomes, and that these were arranged in the 

 equatorial plate of the spindle. When the spindle reaches its definite 

 position these chromosomes divide and pass toward opposite poles of 

 the spindle if the egg has been fertilized; otherwise they remain in- 

 definitely in the equatorial plate. 



The form of the chromosomes in the equatorial plate is liable 

 to great variation, and because of their small size and comparatively 

 large number (sixteen) it is very difficult — and I have found it 

 impossible — to determine the nature of the division which there 



29* 



