48 NATURAL SCIENCE. Jan.. 



definite structure, but is merely an expression of the peculiar sub- 

 stance in the archoplasm, and has attractive power on the chromo- 

 somes. Ed. van Beneden considered .that the spindle threads were 

 contractile, and fixed into the centrosome as a fulcrum. More careful 

 examination has shown that the centrosome is invariably separated 

 from the striated archoplasmic sphere by a light area ("heller Hof " 

 of Boveri), in which there are no fibres or striations. It is, therefore, 

 best to dismiss the idea of contractile threads, and to consider that 

 the chromosomes move by virtue of their own activity along the 

 threads of the spindle. That they do so under some sort of chemio- 

 tactic influence derived from the centrosome is also very probable, 

 nay, almost certain. Pick has, in his paper on the fertilisation of the 

 egg of the axolotl, shown that the spermatozoon, after entering the 

 ovum, turns completely round, so as to bring the " Mittelstiick" foremost 

 in the direction of the centre of the ovum. The latter proceeds 

 towards the egg nucleus, while the former follows a short distance 

 behind by means of amoeboid movements. If in this case a centro- 

 some can apparently exercise such a strong attractive power on the 

 sperm nucleus, which is nothing more than a mass of chromatin, why 

 should it not have a similar influence on the chromosomes in a nuclear 

 division ? 



As opposed to this view it may, however, be asked. What is the 

 use of the spindle if the centrosome can act so powerfully on the 

 chromosomes ? In answer to this, it is possible to imagine that, 

 owing to the necessity for perfect halving of the chromosomes, a 

 definite path is necessary for each chromosome to move along in order 

 to prevent confusion. Haecker believes that he has discovered the 

 actual product which acts chemiotactically on the chromosomes. 

 Figs. II and 12 shoAv this. Fig. 11 represents a centrosome sur- 

 rounded by its archoplasmic sphere at the time when the equa- 

 torial plate is just being formed. It will be noticed that the " heller 

 Hof"(/?/?.)is large, and that the centrosome has somewhat of avesicular 

 appearance. Fig. 12 is drawn at the moment when the chromosomes 

 have begun to migrate towards the poles. It will be seen that the 

 centrosome now appears as a homogeneous granule, while between it 

 and the " heller Hof" is a zone of a staining semi-fluid consistence. 

 This Haecker believes to be the contents of the centrosome, which 

 are thrown out, and exert the attracting influence on the chromo- 

 somes. 



Haecker's work was chiefly carried out on Cladoceran eggs. His 

 views must be considered as yet to be purely speculatory, especially 

 as they do not appear able to fit in with facts described by other 

 observers. 



{b) The fate of the Nucleolus.— The word " nucleolus " has, unfor- 

 tunately, to do duty for at least three distinct components of the 

 nucleus, viz. : — (i.) The true nucleoli, which appear to be part of the 

 achromatic network, and are unaffected by ordinary staining fluids. 



