158 INTRODUCTION TO CYTOLOGY 



those forms with more or less permanent asters. The centriole at each 

 pole divides, usually before the anaphase begins and often even earlier. 

 In case the aster about it does not disappear at the close of mitosis, a 

 new amphiaster may differentiate about the daughter centrioles in the 

 midst of the old aster. The latter rarely divides but usually degenerates 

 as the new amphiaster develops within it.^ 



A mass of spindle substance, sometimes surrounded by chondriosomes 

 in the cytoplasm, remains for some time between the chromosome groups 

 and the young nuclei w^hich they form. In animals this substance plays 

 no very conspicuous part in cytokinesis. Granules may be differentiated 

 at the equatorial region, forming the so-called "mid-body," but the 

 division of the cytoplasm is ordinarily brought about by the development 

 of a cleavage furrow, as will be described in the next chapter. 



Very characteristic amphiastral figures are developed in the ascomy- 

 cetes^" (Figs. 87, 88). The centrosome, which in some ascomycetes is 

 discoid, lies against the nuclear membrane. As mitosis begins, an aster 

 usually develops in the cytoplasm about the centrosome, and the latter 

 divides to form two daughter centrosomes. The central spindle, if 

 formed at all, does not persist. From each of the daughter centrosomes, 

 which begin to move apart along the nuclear membrane, a group of 

 "fibers" extends into the nucleus. This appearance may be due to the 

 fact that the centrosomes here mark regions where there is an active 

 material interchange between nucleus and cytoplasm (Harper, 1919), 

 the result being the orientation of prophasic karyolymph to form the 

 spindle structure. The centrosomes finally reach opposite sides of the 

 nucleus, and the two groups of fibers become arranged in the form of a 

 spindle extending through the nucleus with the chromosomes at the 

 equator. The spindle may occupy nearly all of the nuclear volume or 

 only a small portion of it. The nuclear membrane commonly remains 

 intact until the chromosomes approach the poles at anaphase. It may 

 then disappear, allowing the nucleolus, which has remained visibly 

 unchanged, to escape into the cytoplasm. Between the two densely 

 packed daughter chromosome groups there extends a long strand of 

 spindle substance; this disappears as the daughter nuclei reorganize. 



Cytokinesis in fungi is commonly brought about by a cleavage furrow 

 which is independent of the achromatic figure. After the final mitosis in 

 the ascus the astral rays curve around and in some way become involved 

 in the formation of the ascospore membrane. According to Harper, the 



9 See Wilson (1925, p. 680). 



'« Harper (1895, 1897, 1899, 1905), Faull (1905, 1912), Maire (1905a), Guillier- 

 mond (1904, 1905, 1911), Fraser (1908), Eraser and Brooks (1909), Fraser and Wels- 

 ford (1908), Claussen (1912), W. H. Brown (1909, 1910a, 19116), Bagchee (1925), 

 Carruthers (1911), Schultz (1927), Tandy (1927), Eftimiu (1927, 1929), Gwynne- 

 Vaughan and Williamson (1930, 1931, 1932). 



