190 



INTRODUCTION TO CYTOLOGY 



the karyosome elongates and divides by constriction at the time of 

 nuclear division. Often it forms a dumb-bell-shaped structure, about 

 the middle of which the chromatic granules or chromosomes are grouped 

 and separated into two masses as the nuclear membrane remains intact 

 (Fig. 111). Frequently it does not divide until the elongated chromo- 

 somes have reached the poles. The chromosomes lying parallel to 

 the axis of the division figure may appear to break in two at the middle, 

 but it has been shown by Hall and others that in the prophase they 

 are split and opened out from one end, the supposed transverse division 



I 





Fig. 111. — Mitosis in Heteronema acus. (After Loefer, 1931.) 



being merely the completion of the separation at the other end.^^ It is 

 claimed that in Euglena the split is present in the preceding telophase.^* 

 While these changes are in progress the centrosome and motor elements, 

 if present, become doubled in number in various ways to form the cor- 

 responding elements of the daughter cells^^ (see p. 223). 



In the Plasmodiophorales^^ the chromosomes form a ring at the equa- 

 tor of the mitotic figure; as this ring divides and separates to the two poles, 

 the karyosome it encompasses does likewise. In the green alga Clado- 

 phora glomerata, the nucleolus elongates and divides as the split chromo- 

 somes separate (Fig. 112). The nuclear membrane remains intact 

 throughout mitosis, and at certain stages the whole figure bears a striking 

 resemblance to those described above." The behavior of the nucleolus 



23 R. P. Hall on Menoidium (1923), Oxyrrhis (1925a) and Ceratium (1925fc); Hall 

 and Powell (1928) on Peranema; Loefer (1931) on Heteronema. 



24Tschenzoff (1916), Tannreuther (1923), Ratcliffe (1927). 



2^ Kofoid and Christiansen (1915), Kofoid and Swezy (1915 et seq.), Swezy (1915, 

 1916, 1922), Wenrich (1921), Boeck (1917), Hall (1923, 1925a6), Bunting and Wenrich 

 (1929), Kater (1929). 



26 S. Nawaschin (18996), Winge (1912) on Sorodiscus, Cook (1928) on Ldgniera, 

 Home (1930) on Spongospora, Milovidov (1931) on Plasmodiophora. For other 

 accounts of mitosis in myxomycetes, see Strasburger (1884a), Harper (1900a), Jahn 

 (1904, 1908, 1911), Olive (1907), Prowazek (1905), Kranzlin (1907), and Skupieiiski 

 (1928). 



2^ T'Serclaes (1922). For other recent accounts of mitosis in green algse, see 

 Czurda (1922a6) and Geitler (1930) on Spirogyra, Foyn (1929) on Cladophora and 

 Ulva, Hartmann (1921) on Eudorina, Higgins (1930) and Schussnig (1930) on Clado- 

 phora, Kretschmer (1930) and Ohashi (1930) on (Edogoniu7n, Mundie (1929) on Vauch- 

 eria, Peterschilka (1922) on Mougeotia, Schussnig (1928a6) on Caulerpa, and M. 

 Williams (19256, 1926) on C odium and Vaucheria. 



