24: INTRODUCTION PKOTOPLASM CELL 



tory study, at least as an approach to the subject, is that shown in the 

 root tip of the dogtooth violet (Fig. 30, a to n). The cells and the mi- 

 totic figures are here so large that all the major details can be easily 

 recognized by use of the usual dry high power lenses of the microscope. 

 (1) Prophase. This stage can again be subdivided into that (a) 

 of the resting nucleus; (b) of the nucleus with dose spireme; (c) of the 

 nucleus with the loose spireme; and (d) of the segmented spire in > 

 Coincident with these nuclear changes, the centrosome in animal cells 

 divides into two (diplosome) ; these moieties move apart toward opposite 

 poles of the nucleus and build a spindle (amphiaster) between them- 

 selves. Meanwhile the nuclear membrane begins to disappear, only a 

 remnant distant to the achromatic spindle persisting at the end of the 

 prophase. In the root tip cell of the dogtooth violet and in plant cells 

 generally, the spindle appears in less conspicuous fashion than in animal 

 cells. A centrosome is apparently lacking. The first indications of the 

 spindle are the polar caps of faint radiations which grow medially to build 

 the spindle. The resting nucleus (Fig. 30, a) is characterized by a 

 random, granular, nuclear reticulum with net-knots and one or several 

 nucleoli. This reticulum becomes changed into a delicate, deeply chro- 

 matic, probably continuous, close spireme (Fig. 30, b). By process of 

 shortening and thickening, this changes into the loose spireme stage 

 (Fig. 30, c). The nucleoli have meanwhile contributed chromatic sub- 

 stance to the spireme, but may persist for some time longer as achro- 

 matic, ultimately fragmenting or dissolving, bodies. On closer inspection 

 the close spireme is seen to consist of a series of granules (chromomeres) ; 

 during the loose spireme stage these become split, thus giving rise to 

 a double row of granules. The loose spireme passes into the succeeding 

 stage (segmented spireme, Fig. 30, d) by transverse segmentation inco a 

 number of rods or chromosomes. At this stage all indication of chro- 

 momeres is generally again lost, the chromosomes appearing as compact, 

 deeply staining rods of various shapes. 



The number of chromosomes is believed to be constant for all cells of 

 a species. This belief rests upon data of actual counts in various insects and 

 other lower animals and certain plant forms. Here the number is rela- 

 tively small, and the individual chromosomes are large and can in conse- 

 quence be readily counted. (Certain qualifying statements must be made 

 in the chapter which includes a discussion of sex determination.) Attempts 

 have recently been made to throw doubt upon the matter of a .specific chro- 

 mosome constancy, but it is only fair to note that these attempts have 

 dealt with relatively unfavorable material, where exact chromosome counts 



