224 
Allen . — Nuclear Division in the 
(on the assumption that the chromosome substance is the bearer of the 
hereditary characters of the organism) the basis for two separate and more 
or less different sets of individual qualities. A considerable amount of 
evidence has been brought forward, especially by Riickert (’95) and 
Hacker (’97, ’02), tending to show that not only do the parental chromo- 
somes retain their individuality, but, at least in the earlier divisions, present 
a distinct separation into two groups ; and Hacker finds that at various 
stages in the ancestry of the germ-cells, even down to the appearance 
of the primary oocyte and primary spermatocyte, the nuclei show, by their 
two-parted, two-lobed or flattened form, and by the possession of two 
nucleoles and occasionally of two spirems, evidences of their twofold nature. 
On the other hand, Moenkhaus (’04), who succeeded in obtaining hybrids 
between two species of fishes having chromosomes differing in form and 
size, finds that, although the chromosomes retain their morphological 
individuality, they lose their original grouping after the first two cleavage 
divisions. The fact that in these later stages Moenkhaus found bilobed 
and binucleolated nuclei, as well as a grouping of chromosomes into two 
(mingled) groups, shows that while the phenomena discussed by Hacker 
may result from the continued independence of the two parental sub- 
stances, they are not sufficient evidence for the acceptance of the view that 
these substances are isolated in different regions of the nucleus. 
It should be added that there is nothing in the evidence now at hand 
which absolutely negatives the possibility of an interaction between the 
two sets of chromosomes present in each somatic nucleus, by which during 
the life of the organism some chemical or other change is effected in the 
constitution of the chromosomes themselves. All that seems reasonably 
certain is that the chromosomes remain as distinct entities, and that 
(at least in the direct ancestry of the germ-cells) they undergo no visible 
modifications except those attendant upon their alternate growth and 
division from one cell generation to another. 
In connexion with the constancy of the chromosome number and 
the persistence of individual chromosomes, the question arises whether 
all the chromosomes in a given nucleus are essentially similar to one 
another, or whether they differ in structure and function. That the 
chromosomes received by the organism from either parent are sufficient 
for its complete development is shown by Boveri’s (’89) experiments on 
the fertilization of enucleated egg-fragments, and by recent work on 
artificially-induced parthenogenesis (Loeb, ’99 ; Wilson, ’01). The nucleus 
of the sexually-produced individual, then, contains at least two complete 
sets of hereditary substances, either set capable of inducing the development 
of a complete individual. Boveri’s (’02) experiments on double-fertilized 
sea-urchin eggs, which divide simultaneously into three or four cells contain- 
ing varying numbers of chromosomes, go far toward showing a qualitative 
