286 



somes of the next generation. The reticulum of threads and granules 

 is then drawn out into an irregular spindle, the central spindle (Dia. 

 B, 7). In the meantime the outlines of the mother centrosome have 

 faded away and the new amphiaster migrates out of the sphere and 

 lies free in the cytoplasm. In this case therefore as in that of the 

 maturation, the daughter centrosomes and central spindle arise within 

 the mother centrosome. 



There is in the cell cycle a remarkable parallelism between the 

 nucleus and the centrosome in these molluscau eggs: 



1) Both arise from small granules derived from a mother struc- 

 ture, the central corpuscle in the one case, the chromosomes in the 

 other. 



2) In both only a small fraction of the mother structure (nucleus 

 or centrosome) passes into the daughter's one. 



3) Both grow rapidly, absorbing achromatic material from the 

 cell body (the Diastole). 



4) In both the outer membrane is dissolved and the contents are 

 set free in the cell body, only the granules mentioned above (1) per- 

 sisting to form the daughter structures (the Systole). 



5) Both undergo similar changes in their staining reactions, pas- 

 sing from a condition in which they are uniformly chromatic to one 

 in which they are almost entirely plasmatic in reaction ; finally portions 

 of each again become chromatic. 



6) When they have reached their greatest volume both are pro- 

 portional in size to the size of the cell in which they are found. 



Finally we may dra\v the following conclusions as to the pecu- 

 liarities of the molluscan centrosome: 



1) It corresponds to the couche medullaire and the cor- 

 puscle central of Van Beneden. 



2) The medullary zone is bounded by a remarkably thick and 

 densely staining layer which at all stages sharply separates it from 

 the surrounding sphere. 



3) The centrosome undergoes unusuall growth during mitosis. 



4) The entire amphiaster of one generation arises \vithin the 

 centrosome of a preceding generation. 



Spheres. In the prophase of the various cleavages the spheres 

 (couche corticale of Van Beneden) are small areas free from 

 alveoles, lying immediately around the centrosomes; in the metaphase 

 they are larger and their substance diffuses along the astral rays ; in 

 the anaphase they become much larger and are more regularly sphe- 

 rical in outline. During the prophase and metaphase astral rays can 



