IOWA ACADEMY OF SCIENCE 
109 
definite loops, which encircle the nucleus in somewhat regular 
fashion. The ribbon finally breaks up into sixteen distinct chromo- 
somes, the breaks taking place at the apices of the loops. For a 
time the chromosomes frequently remain arranged equatorially 
about the nucleus, as in figure 31, but the arrangement is usually 
more irregular, as in figure 30. The beginning of the spindle is 
usually coincident with the breaking up of the chromatin ribbon 
and disappearance of the nucleolus, but such is not invariably the 
case. Frequently the nucleolus persists after the separation of the 
chromosomes, and in a few cases the spindle was observed to have 
become quite well defined before the disappearance of either the 
nuclear membrane or the nucleolus. As a general rule, however, 
it may be said that the breaking up of the chromatin ribbon is 
quickly followed by the disappearance of the nucleolus and the be- 
ginning of the spindle and the latter is quickly followed by or co- 
incident with the disappearance of the nuclear membrane. 
The prevalent opinion of botanists that the centrosome does not 
exist as an organ of the cell in vascular plants is so well accepted, 
and apparently so well grounded in fact that he would be a bold 
man who would dare dispute it. Certainly no such attempt will be 
made by myself, who am in the possession of no mass of data ade- 
quate to the formation of an independent conclusion upon such a 
subject. I have merely endeavored to reproduce as accurately as 
possible the appearance of the preparations I have studied, making 
no dogmatic assertion as to their meaning, but leaving others to 
attach any interpretation that may appear to them to be reasonable. 
The attraction spheres figured in numbers 35 to 40, are very doubt- 
ful. They appear very faintly in the original sections, and may 
be a part of the cytoplasm, which is granular and contains many 
bodies that might, in the proper position, be mistaken for centro- 
spheres. Possibly the structures figured are merely such granular 
cytoplasmic bodies. On the other hand, in figures 33 and 34 appear 
distinct hyaline areas on the nucleus, each containing a small but 
deeply stained and well defined body, which appears to correspond 
closely to the centrosome, as it has been described. The nuclei in 
question are in very early prophase of division, which lends color 
to the supposition that the structures in question are in reality 
attraction spheres. If they are not attraction spheres, the question 
as to what they are is a rather difficult one to answer. 
In the mother star stage, the chromosomes adhere to the spindle 
by their middles, each one assuming the shape of a V. Splitting 
begins at the apex of the V and continues outward to the ends. The 
daughter star and daughter skein stages and formation of the cell 
plate exhibit no unusual features. 
