The maturation and fertilization of the egg of Cerebratulus. 463 



centrosome, without conspicuous centrosphere. So far as I have been 

 able to determine, however, the aster-fibres are in no case attached 

 directly to the centrosome itself as is described by Kostanecki & 

 WiERzEjSKi in Physa. Even in the most minute asters which it is 

 possible to find (namely, those which develop within the centrospheres 

 of the cleavage-asters, and which show only the slightest traces of 

 radiations) the fibres are continuous, not with the centrosome, but 

 with a delicate shell of protoplasm in which the centrosome is im- 

 bedded. At times, it is true, this shell becomes so thin as to be 

 barely visible, yet indications of it may always be seen except in those 

 cases in which the centrosome loses its aster, and comes to lie directly 

 in the general cell- cytoplasm. Instances of such centrosomes without 

 surrounding centrospheres are to be found in the outer poles of the 

 polar spindles after these have lost their asters. This is also true 

 of the centrosomes in the degenerating asters of the sperm-nucleus 

 just before the union of the germ-nuclei. 



As the aster increases in size, the centrosphere enlarges also, but 

 the centrosome changes but little from its original size (that is the 

 size when first distinguishable) no matter how large the aster and 

 centrosphere may become. When the centrosphere has attained a 

 considerable size, as in the cleavage -asters, it can be seen that the 

 aster-fibres pass directly into its substance , and in some cases may 

 be followed nearly, though not quite, to the centrosome. 



In Cerebratulus, the centrosome is at all times (except those cases 

 mentioned above) marked off from the surrounding centrosphere with 

 the greatest clearness ^). It is never anything more or less than a 

 sharply-outlined granule staining intensely black with Bordeaux and 

 iron - haematoxylin , while the surrounding centrosphere stains only 

 faintly red. On reaching the centrosphere, the aster-fibres diminish 

 greatly in size, and stain red, even when tliey are black 

 elsewhere. As the fibre is followed closer to the centrosome the 

 granules of which it is composed are found to be much smaller, and 



1) In certain preparations, however, the centrosome seems to be 

 either entirely dissolved away or to be affected in such a manner that 

 it loses its affinity for stains, for no trace of it can be found. Wilson 

 has found this to be true of the corrosive-acetic killing fluid on the 

 eggs of the sea-urchin. It w^as with corrosive-acetic , however , that 

 Hill (11) found the centrosomes in the cleavage asters of Sphaerechinus, 

 and I have found that when but 2 — 5 ^/q of acetic acid is used the 

 centrosomes are often perfectly preserved. 



