OUTLINE OF INDIRECT DIVISION OR MITOSIS 49 



cveji. Thus, in some of the sharks the number is 36; in certain 

 gasteropods it is 32 ; in the mouse, the salamander, the trout, the Hly, 

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

 number 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 Artcmia it is 168.1 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. 61). The even number of chromosomes is a most interestins: 

 fact, which, as will appear hereafter (p. 135), 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, 

 consisting of true chromatin, probably enter into the formation of the 

 spireme-thread. True nucleoli seem to dissolve and disappear, or in 

 some cases are cast out bodily into the cytoplasm, where they degen- 

 erate and have no further function. Whether they ever contribute 

 to the formation of chromosomes is uncertain. 



{b) The Aviphiastcr. Meanwhile, more or less nearly parallel with 

 these changes in the chromatin, a complicated structure known as the 

 aviphiastcr {Yo\, 'yj) makes its appearance in the position formerly 

 occupied by the nucleus (Fig. 19, B-F). This structure consists 

 of a fibrous spindle-shaped body, the spindle, at either pole of which 

 is a star or asfer formed of rays or astral fibres radiating into the sur- 

 rounding cytoplasm, the whole strongly suggesting the arrangement 

 of iron filings in the field of a horseshoe magnet. The centre of each 

 aster is occupied by a minute body, known as the ccntrosome (Boveri, 

 '88), which may be surrounded by a spherical mass known as the 

 centrosphcre (Strasburger, '93). As the amphiaster forms, the chro- 

 mosomes group themselves in a plane passing through the equator of 

 the spindle, and thus form what is known as the equatorial plate. 



The amphiaster arises under the influence of the centrosome of the 

 resting-cell, which divides into two similar halves, an aster being 

 developed around each while a spindle stretches between them (Fig. 

 19, A-D). In most cases this process begins outside the nucleus, but 

 the subsequent phenomena vary considerably in different forms. In 

 some forms (tissue-cells of the salamander) the amphiaster at first lies 

 tangentially outside the nucleus, and as the nuclear membrane fades 

 away, some of the astral rays grow into the nucleus from the side, 

 become attached to the chromosomes, and finally pull them into posi- 

 tion around the equator of the spindle, which is here called the cen- 

 tral spindle (Figs. 19, D, F\ 21). In other cases the original spindle 



^ For a mure complete list see p. 154. 



