The maturation and fertilization of the egg of Cerebratulus. 449 



outline. The luicleus grows by an enormous increase both of its 

 fluid matrix and of its chromatic substance. There is considerable 

 variation in regard to the size which the egg-nucleus may eventually 

 attain before its fusion with the equally large sperm-nucleus, but in 

 general its diameter is about one-seventh of that of the egg itself. 

 At this stage it shows a regular network of chromatic granules of 

 various sizes, and usually one or more small nucleoli^). Meanwhile 

 the radiations from the inner pole of the second spindle, which have 

 lingered about the forming egg-nucleus become less and less regular 

 and distinct (Figs. 12, 23—25), until they finally disappear completely. 

 The formation of the sperm-nucleus takes place in a similar 

 manner. The elongated sperm-head becomes much shorter and thicker 

 soon after its entrance into the egg. It then apparently rotates through 

 an angle of about 180"^ as Wilson (24) has so fully described for 

 Toxopneustes. The solid, homogeneous mass of chromatin increases 

 rapidly in size, and soon becomes broken up into a number of irre- 

 gular lumps. A distinct nuclear membrane now appears, and the 

 lumps of chromatin become smaller and more numerous, and are 

 separated by a quantity of apparently fluid matrix (Figs. 42—45). 

 The outline of the nuclear membrane at this period is usually very 

 irregular. As this sperm-nucleus approaches the centre of the egg 

 its form becomes more regular and its chromatin arranged in a net- 

 work (Figs. 23—25) instead of lying irregularly against the nuclear 

 membrane as previously. Several small nucleoli commonly make their 

 appearance before this stage. When near the egg-nucleus the outline 

 of the sperm-nucleus becomes smooth and regular except as it may 

 throw out pseudopod-like processes. Its chromatin has enormously 

 increased in mass and has become arranged in a regular network of 

 chromatic granules, supported by a delicate linin-network. 



The Disappearance of the Sperm-asters. 



Let us return now to the achromatic structures. The radiations 

 about the egg-nucleus have entirely disappeared at a stage when the 

 germ-nuclei lie side by side. The radiations of the sperm-asters, on 

 the other hand, have just reached their maximum development although, 

 as was shown above, their centrospheres have disappeared leaving the 



1) The egg-nuclei of Micrura caeca at this stage show several 

 (3 — 7) nucleoli of considerable size. These disappear completely at the 

 appearance of the cleavage asters , as has been described for other 

 animals. 



