1900 | DEVELOPMENT OF THE POLLEN GRAIN 85 
the ribbon. Coincident with this, there is a thickening anda 
contraction in these looped regions. In some cases this thicken- 
ing seems to be uniform; but generally there are very definite 
chromatin masses. Gradually these chromatin masses increase 
in size, and most of the chromatin disappears from the connect- 
ing regions. 
The above changes lead to a condition very striking in appear- 
ance. At many places where thickening has occurred, there 
are several, usually about four, distinct chromatin aggregations 
(fig. 3). These are usually arranged in such a position as to 
suggest a division intwo directions; hence a tetrad in the sense 
of the Freiburg school of zoologists. In many other instances 
there are distinct links open at one end, although the chromatin 
is in masses in these links. Xand Y forms are much less common 
(fig. 4). A study of the changes leading to these conditions, 
however, seems to exclude the probability of tetrad-formation. 
Moreover, when viewed in the plane of the link there is indica- 
tion of a longitudinal division in the linin connections between 
the chromatin masses, but never very marked. It is probable 
that the ribbon bends upon itself abruptly in the regions of the 
thickened chromatin masses to make the link forms, and that 
diverging forms are to be ascribed to a partial separation and 
reunion at the bend. If we accept this interpretation each chro- 
matin group represents two chromosomes attached end to end 
and bent upon each other, with only a local indication of longitu- 
‘dinal fission, giving ultimately an effect such as others have 
described for Lilium; also for Helleborus, Podophyllum, Iris, 
and other plants. As these loops or chromatin groups increase 
in size, the indication of chromatin granules in the connecting 
linin diminishes. During early segmentation the prominence of 
these linin connections seems paralleled in certain Hepaticae 
studied by Farmer.3 
During the aggregation of the chromatin into definite groups, 
important changes occur in the nucleolus. From the time when 
the large nucleolus is in close connection with one or more 
3 Loe. cit., p. 8, figs. 5,6, 41. 
