The maturation and fertilization of the egg of Cerebratulus. 457 



former (32) finds abundant proof that they come exclusively from the 

 egg, while the latter (17) is equally positive that they are of male 

 origin. 



The Cleavage -Spindle. 



While the sperm-asters and centrosomes are disappearing the 

 germ-nuclei attain their full size and development, and come to lie 

 side by side in the region between the centre of the egg and the 

 point where the polar bodies were given oft. In nearly all cases the 

 germ- nuclei fuse completely to form a single body^). The chromatin 

 of the two may remain more or less separate, but is more commonly 

 united into a single network. In the act of fusion the germ-nuclei 

 often send out one or more blunt processes which somewhat resemble 

 the pseudopodia of Rhizopods. Where these pseudopod-like processes 

 come in contact, the delicate nuclear membranes between them break 

 down (Fig. 29) and the contents of the two nuclei flow together to 

 form the cleavage-nucleus (Fig. 30). This nucleus elongates somewhat, 

 and moves to a position of symmetry in the egg, with its longer 

 axis a right angles to the former axes of the polar spindles. At this 

 time, or slightly earlier, the centrosomes of the future cleavage-spindle 

 appear. When first seen they usually occupy positions very near the 

 extremities of the elongated nucleus, although, as stated above, the 

 centrosomes occasionally appear even before the com- 

 plete fusion of the germ -nuclei, in which case they occupy 

 positions in the angles between the nuclei. Each centrosome is the 

 centre of a few delicate radiations. These rapidly increase in number, 

 size and length, and at the same time the nuclear membrane in their 

 immediate vicinities begins to disappear. In nearly all of the numerous 

 eggs which I have examined in this stage the nuclear membrane was 

 already interrupted at the two points occupied by the asters. The 

 centrosomes then lie in such a position that if we were to join the 

 interrupted ends of the membrane in a section the line would pass 

 through the centrosome. This does not indicate that the centrosomes 



1) There is considerable variation in this respect, which reminds 

 one of the condition in Ascaris and other animals where the germ- 

 nuclei may or may not unite, according to circumstances. In Micrura 

 caeca (Fig. 50) it often happens that the cleavage - spindle is formed, 

 as in many other animals , without the germ-nuclei even coming in 

 contact. 



30* 



