OUTLINE OF INDIRECT DIVISION 6/ 



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

 ments, forming a segmented spireme. In either case it ultimately 

 breaks transversely to form the chromosomes, which in most cases have 

 the form of rods, straight or curved, though they are sometimes spher- 

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

 form of rings. The staining-povver of the chromatin is now at a maxi- 

 mum. As a rule "the nuclear membrane 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 nucleus becomes continuous with the surrounding cytoplasm 



(Fig. 25, A£,/^V 



The remarkable fact has now been established with high probability 

 that eveiy species of plant or animal has a fixed and characteristic niDH- 

 ber of cJiromosomes, zvhicJi regularly recurs in the division of all of its 

 cells ; and in all forms arising by sexual reproduction the number is 

 even. 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 - 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 168.^ 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. %'j). 

 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. 



The 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 {ViQ.\m\Q.x, "^6, and Moll, '93 ) or .-^r//- 

 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 nucleoli or plasmo- 

 somes sooner or later disappear ; and the greater number of observers 

 agree that they do not take part in the chromosome-formation. In a 

 considerable number of forms {e.g. during the formation of the j^olar 



1 The spireme-formation is by no means an invariable occurrence in mit»)sis. In a consid- 

 erable number of cases the chromatin-network resolves itself ilirectly into tlie 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 Flemmino believes the number in man to be considerably greater than 16. 

 ^ For a more complete list see p. 206. 



