Chromosomes in Living Organisms . 31 1 
the reduction of the number of chromosomes in the mother- 
cells of the spores. Hybrids of similar origin resemble each 
other, in the first generation, because the chromosomes of 
both parents persist side by side in all the nuclei of these 
hybrids, and affect the processes of development in a definite 
manner. The offspring of these hybrids behave differently, 
doubtless because there is a fusion of the parental chromosomes 
and a corresponding reduction in the number of the ids in the 
process of development of the spore-mother-cells (mother-cells 
of the pollen and of the embryo-sac). Hence interference 
becomes more active, and renders possible a difference in the 
sexual cells ; and the union of these diverse cells in the sexual 
act is the starting-point of diversity in the progeny. It is 
conceivable that this diversity is due to the influence of 
isolated ids, of ids derived from one or other of the original 
parents, but remaining unfused. It may be further suggested 
that the continued production of unchanged progeny by 
hybrids is only possible in those cases in which the chromo- 
somes and the ids of the original parents retain their primitive 
equivalence even after reduction and fusion have taken place. 
The process of reduction of the number of the chromosomes 
by half takes place, in the Muscineae, Pteridophyta, and 
Phanerogamia, in the spore-mother-cells, that is, at the close 
of the generation developed from the fertilized ovum ; but in 
the lower Cryptogams, where the cell produced by the sexual 
act does not give rise to a definite organism representing the 
asexual generation, the reduction probably takes place on the 
germination of this cell. The attempt has been made to give 
a phylogenetic explanation of this reduction in the number of 
the chromosomes, and it has been regarded as a reversion of 
ontogeny to the point of origin. \ The phenomenon under 
consideration is essentially that of the return of the most 
highly organized plants, at the close of their life-cycle, to the 
unicellular condition : in one word, it is the repetition of 
phylogeny in ontogeny. This explanation seems to me to be 
likewise the only one admissible in the case of animals. It is, 
however, an altogether different question, whether or not the 
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