The maturation and fertilization of the egg of Cerebratulus. 439 



he considers still within the nuclear membrane. The aster-fibers about 

 it, however, extend far out into the substance of the cytoplasm. It 

 is difficult to understand how such a centrosome. while still separated 

 from the cytoplasm by a very definite nuclear membrane, can exert 

 its influence on the cytoplasm in such a manner as to form a well 

 developed astral system — as the figure represents — about it. If 

 this observation be correct it is not only strong evidence, as Klinckow- 

 STRÖM observes, of the intra-nuclear origin of the centrosome, but it 

 also endows this organ of the cell with a power quite beyond that 

 which it has previously been supposed to possess. 



Wherever the maturation centrosomes may originate in Cere- 

 bratulus it is certain that they appear simultaneously (though, as 

 stated above, they are widely separated and apparently independent 

 of each other) and their history can be followed uninterruptedly until 

 the second polar body has been given off, when they are again lost 

 sight of. The asters about the centrosomes increase rapidly both in 

 the number and in the length of their fibres. Many of the fibres 

 which grow towards the nuclear membrane may be seen to come into 

 direct contact with it. As the indentation becomes deeper and deeper 

 the fibres elongate to the same extent. Or, conversely, as the fibres 

 elongate in the direction of the nucleus the nuclear membrane is 

 pushed in an equal distance. As the indentations continue to grow, 

 they become much more irregular and ragged in outline often showing 

 deep recesses into which one or more long aster-fibres may be followed. 

 At the same time, the membrane becomes much thinner and is 

 eventually absorbed entirely away in the vicinity of the two asters 

 (Figs. 4. 5). The cytoplasm now lies in direct contact with the 

 nuclear substance. From this moment the fibres of the two asters 

 extend themselves rapidly into the body of the nucleus, and come in 

 contact with the chromosomes. These have meanwhile been differen- 

 tiated from the nuclear network, as described above, and have mi- 

 grated to that side of the germinal vesicle where the asters are 

 located. Some of the fibres from the two asters pass directly towards 

 each other through the nuclear substance and soon form a spindle 

 between them. The spindle-fibres are undoubtedly formed directly 

 from the substance of the nucleus. All of the chromosomes have 

 become attached to the aster-fibres, and are usually arranged in two 

 groups, of which one belongs to each aster (Fig. 7). Sometimes 

 nearly or quite all of the chromosomes may be attached to one aster, 

 but the final result is the same, for they are all drawn into the 



Zool. Jahib All. Abtli. 1. Morph. 29 



