60 



PRINCIPLES OF ANIMAL BIOLOGY 



The chromosomes differ greatly in size in different organisms, and 

 often in the same cells. Two sizes of chromosomes are shown in Fig. 35, 

 and further differences are represented in Fig. 36. 



The time of duplication or splitting of the chromosomes varies con- 

 siderably. In some cells, as in Fig. 35, the chromosomes do not duplicate 



Fig. 39. Fig. 40. 



Fig. 39. — Vesiculation of chromosomes by formation of protoplasmic film around each 

 chromosome; A early, B late stage. The vesicles do not fuse. 



Fig. 40. — Interphase nucleus of the hellbender, showing the chromosomes distinct and 

 separate as vesicles, in which, however, the chromatin is very diffuse. {From B. G. Smith 

 in Journal of Morphology and Physiology.) 



themselves until they are in the metaphase. In others they are doubled 

 while still in the long ropelike stage before taking their places on the 

 middle of the spindle (Fig. 37), that is, in the prophase. 



Fig. 41. Fig. 42. 



Fig. 41. — Mitosis without centrioles in a cell of the root tip of the hyacinth. {From 

 Dahlgren and Kepner, "Principles of Animal Histology.") 



Fig. 42. — Dividing cell with conspicuous spindle in whitefish embryo. (.Courtesy of 

 General Biological Supply House.) 



The expansion of the chromosomes to form new nuclei at the close 

 of division differs in different animals and plants. In some species there 

 is a very plain formation of vesicles by the accumulation of liquid \\ithin 

 each chromosome (Fig. 38.4). Then the vesicles fuse to form one largo 

 vesicle {B), though it is still quite likely that the chromosomes maintain 



