. Cytology, with special reference to the metazoan nucleus. Cells; Cytology. V CHROMOMERES 137 constant in their arrangement in a given chromosome. Fig. 66 shows one example of the corresponding chromosome from thirteen different individuals. It is taken from the pachytene stage of spermatogenesis, and is therefore bivalent, as indicated by frequent signs of duplicity. The five principal chromomeres are numbered 1-5 and it will be seen how noticeably constant in arrangement they are. This regularity extends also to the smaller chromomeres. For instance, in the segment between Nos. 3 and 4 ther
. Cytology, with special reference to the metazoan nucleus. Cells; Cytology. V CHROMOMERES 137 constant in their arrangement in a given chromosome. Fig. 66 shows one example of the corresponding chromosome from thirteen different individuals. It is taken from the pachytene stage of spermatogenesis, and is therefore bivalent, as indicated by frequent signs of duplicity. The five principal chromomeres are numbered 1-5 and it will be seen how noticeably constant in arrangement they are. This regularity extends also to the smaller chromomeres. For instance, in the segment between Nos. 3 and 4 there are always two small granules of about the same size, while there are never any prominent ones between Nos. 2 and 3. Of course, the constancy is not perfect. A certain amount of variation in the relative sizes of the principal chromomeres, and in the lengths of. /.J i 1 i J ^^ Fig. 66. Various examples of the same chromosome in Phrytwtettix magniis. (After Wemich, , 1916.) A, the cliromosome (bivalent) from the pachytene stage of thirteen different individuals. The principal chromomeres are numbered 1-5. B, successive stages in the contraction of the pachytene chromosome to form the definitive chromosome of metaphase I. the segments separating them, can be observed, as well as in the number and arrangement of the smaller granules in between. This variation may be due to several causes, partly to errors of technique—for instance, distortion by fixing agents, optical effects, etc.—partly to difference in the extent to which fusion of smaller granules to form larger ones has proceeded, but partly probably to real biological variation. The thesis outlined at the beginning of this chapter requires that all genetic differ- ences in organisms should be referred to preceding variation in the idioplasmic elements, and hence it is no more surprising to find variation in homologous chromosomes than in the somatic characteristics of organisms. Wenrich finds indeed, in the c
Size: 2775px × 901px
Photo credit: © Central Historic Books / Alamy / Afripics
License: Licensed
Model Released: No
Keywords: ., bookcentury1900, bookdecade1920, booksubjectcells, bookyear1920
