OUTLINE OF INDIRECT DIVISION 6/ 



its first appearance divided into a number of separate pieces or seg- 

 ments. formmg a segmented spireme. In either case it ultimately 

 breaks transversely to form the chromosomes jjwhich in most cases have 

 thej:orm of rods, straight or curved, though they are sometimes spher- 

 ical or ovoidal, afTQ in certain cases may be joined together in the 

 ^ The staining-power of the chromatin is now at a 



mum. As a rule the nuclear ..mp.mhra.ne meanwhile fades_away and 

 finally disappears, though there are some cases in which it persists 

 more~or less completely through all the phases of division. The 

 chromosomes now lie naked in the cell, and the ground-substance 

 of_the_jnucleus becomes continuous with the surrounding cytoplasm 

 (Fig. 25, A E,Fy 



The remarkable fact has now been established with high probability 

 that every species of plant or animal has a fixed and characteristic num- 

 ber of chromosomes, which regularly recurs in the division of all of its 

 cellsj and in all forms arising by sexual reproduction the nu*n-b<"r ^ 

 evei^. Thus, in some of the sharks the number is 36 ; in certain gas- 

 teropods it is 32 ; in the mouse, the salamander, the trout, the lily, 24 ,-. 

 in the worm Sagitta, 18 ; in the ox, guinea-pig, and in man 2 the num- 

 ber is said to be 16, and the same number is characteristic of the onion.. 

 In the grasshopper it is 12 ; in the hepatic Pallavicinia and some of 

 the nematodes, 8 ; and in Ascaris, another thread- worm, 4 or 2. In the 

 crustacean Artemia it is i68. 3 Under certain conditions, it is true, 

 the number of chromosomes may be less than the normal in a given 

 species ; but these variations are only apparent exceptions (p. 87). 

 The even number of chromosomes is a most interesting fact, which, as 

 will appear hereafter (p. 205), is due to the derivation of one-half the 

 number from each of the parents. 



Thg^nucleoli differ in their behaviour in different cases. Net-knots, 

 or chromatin-nucleoli, contribute to the formation of the chromosomes ; 

 and in cases such as Spirogyra (Meunier T '86, and Moll, '93)0* Acti- 

 nosphcerium (R. Hertwig, '99), where the whole of the chromatin is at 

 one period concentrated into a single mass, the whole chromatic figure 

 thus appears to arise from a "nucleolus." True miclenli or plasmo- 

 spmes sooner or later disappear ; and thejgreater number of observers 

 ^gr^e_thjj_thev__do not take part ip the r rhrnmnsome-formation. In a 

 considerable number of forms (e.g. during the formation of the polar 



1 The spireme-formation is by no means an invariable occurrence in mitosis. In a consid- 

 erable number of cases the chromatin-network resolves itself directly into the chromosomes, 

 the chromatic substance becoming concentrated in separate masses which never form a con- 

 tinuous thread. Such cases are connected by various gradations with the " segmented spi- 

 reme." 



2 Flemming believes the number in man to be considerably greater than 16. 



3 For a more complete list see p. 206. 



