330 Mot tier . — The Development of the Heterotypic 
The behaviour of the chromosomes from their appearance in the 
nuclear plate of the first mitosis to the end of the second or homotype 
division has been described by the writer in an earlier paper (Mottier, ’03), 
and, as a re-examination of that part of the mitotic history has only 
confirmed my former observations, a repetition seems superfluous here. It 
is to be noted, however, that the longitudinal fission of the chromosomes 
during the anaphase or telophase, which was then regarded as a second 
longitudinal fission, is, of course, the first and only such fission in these 
mitoses. 
General Considerations. 
A consideration of the important facts brought out in the preceding 
pages seems to point with a high degree of probability to certain definite 
conclusions, and to indicate more or less definitely several lines of study 
that may lead to fruitful results. In looking over the literature of the 
past ten or twelve years, which deals with the mitotic process in re- 
productive cells among plants, one is impressed with the fact that, at 
different times, the attention of observers has been focused especially 
upon some certain phase of the karyokinetic development. As in most 
problems, presenting many and great difficulties, it has been possible to 
clear up but one point at a time. For a certain period the chief interest 
centred about the centrosome and the development of the spindle, with 
the behaviour of the chromatin taking second place. At present, however, 
the question of centrosome and spindle development may be regarded as 
pretty definitely settled, especially among the higher plants, while con- 
cerning the behaviour of the chromatin there is still much diversity of 
opinion among both botanists and zoologists. The study of nuclear 
division and the sexual process has led to the location of the hereditary 
substance in the nucleus, and the establishment of the doctrine of the 
individuality of the chromosomes, together with their periodic reduction 
to one-half of the number at some definite point in ontogeny. One of the 
most important questions pertaining to these theories was the manner in 
which the reduction is accomplished ; and, if we now admit that this 
problem has found a satisfactory solution, there still remain questions 
suggested by those steps manifested by the nucleus in the evolution of the 
chromosomes themselves, as well as other equally important questions 
that pertain to the manner in which, at least, the more obvious hereditary 
characters are distributed to the respective sex cells by the individual 
chromosomes. 
When, ten years ago, the writer established the fact that the longi- 
tudinal fission of the spirem occurred in the early prophase (i. e. just after 
synapsis) of the heterotypic mitosis of pollen mother-cells, it was not 
considered necessary, even by the leading plant cytologists, to make 
