504 PROCEEDINGS OF THE AMERICAN ACADEMY. 



cross-shaped figures shown by several tetrads in Figures 19^ and 20. 

 The four arms of the cross are aj^proximately equal in length, and 

 each arm is split longitudinally. Thus the chromosome is formed of 

 four chromatids, each of which consists of a continuous chromatin 

 thread extending the length of the two adjacent arms and forming one 

 half of each (Figures 19^', 20). Another common shape assumed by the 

 tetrad is illustrated by the double- V figures (Figure 2()), similar to 

 those observed in other animals by Paulmier ('98), McClung (:00, :02), 

 Sutton (:02), Blackman (:01, :03, :05, :05''), Foot and Strobell (:05), 

 and others. These are obtained by a longitudinal and transverse 

 division of the V-shaped segments, no rotation of the chromatids occur- 

 ring. A third form of the tetrad is shown by chromosomes in Figure 

 22. Here the straight segment undergoes the two cleavages, as in the 

 more common type. It is not converted into the typical cross-shaped 

 figure, however, but remains as a straight band of chromatin consisting 

 of four parts — the chromatids. 



In later stages of the prophase the changes in the tetrads consist 

 principally in a contraction of the arms and a condensation of the 

 chromatin. Thus, in a fairly late prophase (Figure 22) the more common 

 form of the chromosomes is still that of a cross, but the arms of the 

 cross are much shorter and thicker, and in the more advanced tetrads 

 the condensation of the chromatin granules has proceeded so far as to 

 mask the planes of cleavage, these being seen only in the most favor- 

 able cases. In still later stages the chromatin becomes more condensed, 

 and the tetrad goes to the equatorial plate as an apparently homo- 

 geneous body typically of a four-lobed shape. 



Some of the chromosomes in the process of condensation pass through 

 stages similar to those described by Medes (:05) in Scutigera. The 

 chromatin, instead of massing together into one body, becomes con- 

 densed into a number of homogeneous globules, which lie embedded in 

 a more lightly staining substance (Figure 22). These globules later 

 fuse in such a manner as to form a metaphase chromosome of typical 

 appearance. This process will be followed more in detail in the other 

 species of Litliobius, where it is the common method. 



The nuclear changes during the prophase in Litliobius mordax and 

 Lithobius multidentatus will be treated at one time, as they are essen- 

 tially similar in the two forms. In both of these forms all of the 

 chromatin of the nucleus is massed in a single karyosphere, which also 

 contains a considerable portion c f nucleolar material. In addition to 

 this there are plentiful deposits of achromatic material in the nucleus, 

 it being more abundant in Lithobius mordax than in the eastern species. 

 This material, although purely achromatic in nature, plays a part in 

 the evolution of the chromosomes. 



