CHROMOSOME NUMBER AND PAIRS IN AMBYSTOMA 179 
the faithfulness of the description of the conditions under which 
the number of chromosomes was determined. The difficulty 
lies in the necessity of representing on a plane surface chromo¬ 
somes which in the cell occupy several levels. The effect can 
be produced by shading, but at the same time at points where 
chromosomes cross or overlap each other for various distances 
they might create the impression in the drawing that they 
cannot be “ optically traced continuously over their entire 
length.” There are such cases in every drawing. This is 
especially true of the late metaphases of the tail epithelial com¬ 
plexes (e.g., figs. 7, 8) where every chromosome in the cell can 
be clearly and faithfully traced as described above. 
There is also the condition in which parts of the same chromo¬ 
some are so related to one another that their appearance in the 
drawings might create a doubt as to their clearness in the cell. 
Examples of this are represented in figures 6 and 8, chromosome 
‘ a/ in which the two arms of the same chromosome turn abruptly 
upon one another and the appearance might be subject to the 
criticism that there are two different chromosomes involved—a 
portion of one lying exactly upon another with their ends termi¬ 
nating at the same point. Such cases were carefully examined 
end the two arms can clearly be seen to follow into each other. 
In four of this first class of cells there is another condition that 
needs mention. These cells contain one or two chromosomes 
which appear to be broken into two parts (e.g., figs. 19 and 20, /). 
The parts in each case are separated by very short spaces and 
are exactly in line with each other. Della Valle (’09, fig. 11) 
shows two cases of this sort as one chromosome, but discusses 
them (p. 116) as uncertain. That there is a single chromosome 
concerned in each of these cases is further evidenced by the fact 
that there are twenty-one similar cases in other cells of this 
class (e.g., figs. 5, 7, 14 and 15, /) and thirty-five cases in cells 
of class II in which the parts are connected by various amounts 
of chromatin. In some instances the connection is seen as 
faintly staining chromatin, in others as a single or double darkly 
stained thread. 
