Pollen Development in Lactuca. 393 
four pairs, but is probably not tetraploid, since its chromosomes are small 
and may have been derived through a transverse segmentation rather than 
a longitudinal split. A critical study of the chromosomes throughout the 
genus Lactuca would be a valuable aid in determining the relationships 
and phylogeny of its species, and would also throw fresh light on the 
manner in which chromosome numbers and chromosome morphology 
change from species to species. Numerous recent papers have furnished 
preliminary materials for such a study of the phylogeny of chromosomes, 
which will undoubtedly be one of the important future developments in 
cytology. 
Summary. 
In cultivated lettuce (Lactuca sativa , L.) there are .nine pairs of 
chromosomes which form a graded series as regards their length. This is 
true of both the somatic and the meiotic chromosomes. The same number 
is found in L. muralis and probably also in L . Scariola. 
The method of reduction is telosynaptic, the pachynema forming nine 
loops, each of which represents a pair of chromosomes attached at one end, 
the two arms of a loop each constituting a chromosome. These arms 
frequently twist about each other, before or after the spireme segments into 
pairs of chromosomes. As these chromosomes condense they may untwist 
in some cases, but there is some evidence that they frequently break across 
the twists, producing a straight line of separation between the two longitudinal 
halves of a bivalent with crossing over of segments from one chromosome 
to its mate. This differs from the chiasmatypy of Janssens, but furnishes 
a physical basis for genetical crossing-over. 
Synizesis was observed in tapetal nuclei in a few cases. The tapetal 
cells themselves show several peculiarities. They are loosely arranged 
even in the presynaptic stages and they show a variety of types, some 
becoming quadrinucleate and enormously elongated lengthwise of the 
loculus, others remaining short and thick and often binucleate with very 
large nuclei. Some of the latter are scarcely distinguishable from pollen 
mother-cells except by their position. The presence of such transitional 
forms between pollen mother-cells and tapetal cells probably accounts for 
the occasional occurrence of synizesis in such cells. 
With regard to the nature of synizesis or the synaptic knot, it is found 
that the process begins with contraction of the nuclear reticulum and is 
frequently accompanied by the formation of a new precipitation membrane 
surrounding the reticulum. The contraction is accompanied or followed 
by a transformation of the reticulum into a spireme, and in the meantime 
the nuclear membrane undergoes a marked expansion or increase in size. 
The occurrence of synizesis in tapetal cells emphasizes its unique aspect 
as a peculiar physiological condition of the nucleus, but since the pairing 
