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 epithehal 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. Delia 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. 



