340 Mottier . — The Development of the Heterotypic 
somes in plant cells has not been ascertained, and it is apparent at the 
outset that accurate results will be difficult to obtain. That which is of 
greatest theoretical importance is the fact that the number of chromosomes 
in any species is constant. The number, however, whether large or small, 
may be due to purely mechanical causes, but whatever the determining 
factors may be, it is certain that these are fixed by heredity, and the 
difference in the size of the chromosomes in any plant may have no greater 
importance than the difference in the number of chromosomes in different 
plants. The difference in size of the chromosomes in any plant, whenever 
it is found to occur with a high degree of regularity and constancy, is, of 
course, very suggestive, and the writer does not wish to be construed as 
under-estimating the value of such facts, or the usefulness of theories based 
thereon. 
In connexion with any theory of the individuality of the pangens and 
their distribution in reduction, there is another very important fact or 
series of facts to be taken into consideration. In the entire plant kingdom, 
with the one known exception of the genus Finns, the gamete nuclei fuse 
in fecundation while in the resting condition, or when the chromatin in 
each nucleus is in the more finely divided state. It is not possible to 
recognize in the fusion nucleus male and female chromatin, nor is any such 
recognition possible in the nuclei of the sporophyte generation. This 
being the case, what ground is there for assuming that the male and female 
chromatin remain distinct throughout this generation, being merely 
associated in the cells? If there be any rearrangement, exchange, or 
pairing of homologous pangens, why should not all this take place when 
the nuclei fuse, rather than at the close of the cycle when the reduction in 
number of the chromosomes occurs? In the division of the sporophytic 
nuclei there is in many cases, at least, a spirem formed, and this spirem 
separates into double the number of chromosomes, one-half having been 
furnished by the egg and the other half by the sperm. In this spirem we 
must admit that the chromosomes are arranged end-to-end, if we believe 
in the individuality of the chromosomes, for each division is equational. 
Thus arranged, are all paternal chromosomes united end-to-end to form one 
continuous half of the spirem, while the other half is formed by the 
maternal chromosomes ? Or do the parental chromosomes alternate 
with each other in the spirem, or merely become arranged according to 
the rules of chance? If the parental chromosomes are arranged end-to- 
end in somatic cells of the sporophyte, why should they be not so 
arranged in the heterotypic spirem rather than side by side, as those 
who maintain that the double spirem is formed by the union side by 
side of two spirems. In this connexion it is important to remember 
that the univalent chromosomes which show the longitudinal split during 
the anaphase of the heterotypic mitosis do not form a double spirem 
