30 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



/, g, and h. At b, the formation of the arms instead of occurring in the 

 plane of the threads has proceeded in a plane at right angles to it, 

 resulting in the double-V figures first mentioned by Pauhnier ('98). At 

 c, h, the long arms of the cross have been curved around and nearly 

 brought into contact. Such distortions observed in later stages of 

 tetrads result in a figure similar in shape to a seal ring, the point of 

 double cleavage representing the seal and the long arms meeting to form 

 an apparently closed circle. Figure A, e, f, g, are but slight or apparent 

 modifications caused by viewing the tetrads diagonally or in profile. 



#^ 



Fig. A. Fig. B. 



Fig. A. Various stages and modifications of tetrads : a, b, c, early stages in the 

 process of transverse division; d, typical tetrad of niid-propha9e ; e,f,g, h, modifi- 

 cations of the tetrad type. X 1,400 dia. 



Fig. B. Later stages in the history of the tetrad: a, typical cruciform tetrad 

 of late prophase ; b, " double-V " form of chromosome at the same stage ; c, d, 

 successively later stages of the cross figure ; e, J) apparent modifications of tetrad 

 in later prophase ; h, i, typical chromosomes at beginning of metaphase ; rj, tetrad 

 undergoing longitudinal division. X 1,440 dia. 



By later changes the arms of the cross-like figures are much shortened 

 and the divisions of the separate chromatids become very apparent 

 (Fig. B, a, h, c). However, this shortening and condensation continu- 

 ing, these divisions are entirely obliterated, and the chromosome becomes 

 first a granular mass and later apparently homogeneous. The chromo- 

 somes even at this stage vary considerably in shape, as is shown in 

 Figure B, d, e, f, and Figure 29. The typical form is represented by d 

 (Fig. A) and by numerous chromosomes in Figures 28, 29, 30. During 

 the prophase the tetrads of the same nucleus have not developed at the 

 same rate, but, at any given time, the chromosomes of the same cell are in 

 various stages of transformation. When the origin of these elements is 



