Pollen Mother -cells of Lilium canadense. 239 
combination of a complete set of paternal and maternal pangens will result. 
The separation of the chromosomes after this interchange results in the 
appearance of a longitudinal splitting of the chromosomes — the ‘ first 
longitudinal fission/ which has been observed in many cases, especially 
in plants. It is true that in the case of many animals this separation 
seems to be transverse ; but de Vries suggests, as do the Schreiners, that 
after the longitudinal fission the chromosomes may remain temporarily 
in contact by their ends, their final complete separation then producing 
the effect of a transverse breaking. In the case of the offspring of two 
individuals of the same race or variety, the two idioplasms resemble each 
other exactly as to the number and general nature of their pangens ; the 
process just described, then, results in an exchange of like elements, which 
differ from each other only in those minute details which correspond to the 
externally apparent differences between individuals of the same race. In 
the case of a cross between a variety and its mother species, we have to 
deal with two idioplasms which differ from one another only as regards 
a single pangen, which is present in both, but is active in one case, latent 
in the other. Evidently the active and the latent pangen are as capable 
of being exchanged as are any of the other pangens, and the two idioplasms 
may apply themselves to each other as exactly as in the case first 
mentioned. When we come to a cross between two different species, 
however, an exact correspondence between the two idioplasms* is no longer 
to be expected ; the number of pangens, or their arrangement, may differ, 
and so the application of the idioplasms to each other and the consequent 
interchange of pangens is made difficult. If the parents belong to very 
closely related species, this difficulty may not be a serious one ; but as the 
relationship becomes more distant the difficulty increases until the processes 
leading to the formation of the germ-cells become impossible, and the 
hybrid is necessarily sterile. 
Lotsy (’ 04 ) agrees with de Vries that in those cases in which a double 
longitudinal fission of the chromosomes has been observed, each chromo- 
some must be considered as bivalent, and as resulting from a lateral 
apposition of a paternal and a maternal segment. He points out that, 
depending upon the plane in which the split occurs, a longitudinal division 
of such a bivalent chromosome may be either equational or qualitative ; 
and he concludes that of the two longitudinal fissions observed to occur, 
one results in an equation division, the other in a reduction division. He 
points out that the difference between the two planes of separation is 
brought about either by the fact that the successive division-spindles lie 
in planes at right angles to each other, as in the division of animal sperma- 
tocytes, or, if the two divisions are to be in the same plane, as in the 
formation of polar bodies from the animal egg, by a revolution of the 
chromosomes themselves through an arc of 90°. Lotsy considers that, as 
