438 SCIENCE PROGRESS 



no individuality in these units one would have expected an 

 aggregation of these chromosomes into the number characteristic 

 of the species, but such, as we have seen, is not the case. From 

 such observations, Wilson ^ was able to state that " whatever be 

 the number of chromosomes entering into the formation of a 

 reticular nucleus, the same number afterwards issued from it." 



A confirmation of this statement is supplied by those experi- 

 ments in which non-nucleated fragments of eggs have been 

 fertilised by a spermatozoon. Larvae have been reared from 

 such eggs, and in every case the chromosome number has been 

 half that characteristic of the species. 



The retention of a constant number of chromosomes in the 

 nucleus is also illustrated by those cases of natural partheno- 

 genesis in which, through the reduction division of the germ- 

 cells, the chromosomes became reduced to one-half their usual 

 number (haploid condition). This reduced number persists in 

 the germ-cells of such animals, though Nachtsheim * has shown 

 that, in the case of the male bee, they are restored to their 

 original number (diploid) in the somatic cells. 



Finally, in this connection mention may be made of the 

 recent interesting observations on the Crustacean Artemia. 



Artom^ finds that there is a race of this animal from 

 Capodistria in the Adriatic, which reproduces parthenogeneti- 

 cally, and has a constant chromosome number of 84, while a race 

 from Cagliare reproduces sexually, possessing 42 chromosomes, 

 half the size of those of the parthenogenetic race. Similar 

 doubling of the chromosome number has been found and described 

 under the term tetraploidy in several plants.* In the majority of 

 animals and plants the chromosomes cannot be distinguished in 

 the resting stage of the nucleus— which, by the way, is a most un- 

 fortunate term, as, whatever else may rest, the nucleus does not. 



In this so-called resting stage of the nucleus, however, in a 

 few animals and plants it has been found possible to trace the 

 outlines of the chromosomes from one mitosis to the next.^ 

 Though this is an advance on the work of the early cytologists, 

 yet Boveri and Van Beneden showed in Ascaris that at the 

 formation of the spireme in mitosis the chromosomes reappear 

 in the same position they occupied at the formation of the 

 reticulum. By applying the conclusions from such observations 

 to fertilisation, Boveri gave effect to the now famous statement 

 that " in all cells, derived in the regular course of division from 



^ Wilson, E. B., The Cellin Development and Inheritance, p. 296. Second 

 Edition. New York, 1900. 



2 Nachtsheim, H., Arch. Zellforsch., 11, 1913. 



3 Artom, C, Arch. Zellforsch., 7, 1912, p. 277. 



* Gregory, R. P., Proc. Roy. Soc. B., 87, 1914, p. 484. 

 ^ Stout, A. B., Arch. Zellforsch., 9, 1912. p. 114. 



