3o6 



SOME PROBLEMS OF CELL-ORGANIZATION 



('gy) finds that an isolated chromosome, accidentally separated from 

 the equatorial plate (pollen-mother-cells of Hcmcrocallis), may give 

 rise to a small vesicular nucleus which may subsequently divide by 

 mitosis, though it is quite out of relation to the spindle-poles of the 

 preceding mitosis (Fig. 149). Strong evidence of the same character 

 as the last is given by the facts in the heliozoon Acanthocystis, as 

 shown by Schaudinn ("96, 3), the ordinary vegetative cells containing 

 a persistent extra-nuclear centrosome, while in the bud-formation of 

 the swarm-spores a centrosome is formed dc novo, tvitJiout relation to 

 that of tlic inotJicr-ccIl, inside the nucleus of the bud (Fig. 41). 



The strongest case in favour of the independent origin of centro- 

 somes is, however, given by the observations of Mead on Chcetopterus 

 ('98) and the remarkable experiments of R. Hertwig ('95, '96) and 



Fig. 149. — Abnormal mitosis in pollen-mother-cells oi HemerccalUs, showing formation of 

 small nucleus from one or two stray chromosomes and its subsequent division. [JUEL.] 



Morgan ('96, i ; '99, i)on the eggs of echinoderms and other animals. 

 When eggs of Chcctopterjis are taken from the body-cavity and placed 

 in sea-water, a multitude of small asters appear in the cytoplasm, two 

 of which are believed to persist as those of the polar spindle, while 

 the others degenerate (Fig. 150). Mead is therefore convinced 

 that the polar centrosomes arise in this case separately and de iwvo} 

 R. Hertwig showed that when unfertilized eggs of sea-urchins 

 (yStrongylocentrotns, EcJiinus) are kept for some time in sea-water or 

 treated with dilute solutions of strychnine the nuclei undergo some of 



^ A number of other authors {e.g. Griffin, Thalassema, Coe, Cerebraiulus) have likewise 

 found the first polar asters widely separated at their first appearance. On the other hand, 

 Mathews ('95), whose preparations I have seen, finds the polar centrosomes in Asterias 

 close together, and Francotte ('97, '98) has demonstrated that in Cycloporns and Prosthece- 

 rtrus they arise by the division of a single primary centrosome. The same is stated by Gar- 

 diner ('98) to be the case in Polydnxriis. It should be noted, further, that Mead could find 

 no undoubted centrosomes save in the " primary " or definitive polar asters. 



