506 PROCEEDINGS OF THE AMERICAN ACADEMY. 



to correlate the chromosomes at this stage with the type forms described. 

 In later stages this difficulty is rendered still greater by the changes 

 which next take place. These have to do with the condensation of the 

 chromatin by which the tetrads of this stage are converted into the 

 homogeneous chromosomes of the metaphase. The condensation is 

 accomplished by the fusing together of the chromatin globules com- 

 posing each chromatid of the tetrad. It eventually results in the pro- 

 duction of a tetrad made up of four large, deeply stained chromatin 

 globules, each one of which represents a chromatid (Figure 25). 



Tetrads when they have reached this stage are very readily under- 

 stood and correlated with those of an earlier period. The intervening 

 stages (Figures 24, 25), however, proved rather puzzling. These are 

 much more numerous, and were observed before either the foregoing 

 or succeeding ones were found. After all stages were studied, how- 

 ever, the interpretation was simple. All of the chromatin globules 

 belonging to a chromatid do not fuse at once, but this is a process 

 involving considerable time. Thus the chromosomes may contain a 

 variable number of globules, and these often are of v^ery different sizes 

 (Plate 1, Figures 24, 25 ; Plate 2, Figure 26). In other cases chromo- 

 somes were found containing from six to ten spherules of approximately 

 equal size (Figures 24, 25, 27). The latter fact at first inclined me 

 to the belief that the chromosomes here were multiple chromosomes 

 similar to those found by Sindty (:0l) and McClung (:05) in insects, 

 where some chromosomes possess as many as ten chromatids. However, 

 a study of the chromosomes in the late prophase and in the metaphase 

 (Plate 2, Figures 28, 29, 30) rendered this plausible view untenable. 

 The fusing of the chromatic globules continues until all belonging to 

 one chromatid have united to form one large spherule of apparently 

 pure chromatin. 



The structure of the chromosome when it has reached this stage is 

 shown by several examples in Figures 25 and 26. The four chromatids, 

 of about equal size and arranged symmetrically, are enclosed in an 

 irregular homogeneous or granular mass of achromatic material. In 

 the later prophase (Figures 26, 27) this achromatic mantle contracts and 

 becomes more dense, serving to bind the four parts of the chromosome 

 together. Thus the chromosomes at the time of the breaking down of 

 the nuclear membrane are four-lobed structures, which in Lithobius 

 mordax are apparently homogeneous in consistency. 



In Lithobius multidentatus the formation of the tetrad is similar to 

 that in the western species. Apparently the only difference in the 

 chromosomes of the two species is in the smaller size of those in L. 

 multidentatus (compare Figures 26 and 27 with Figure 28) and in the 



