Section 6 — Cytology 



heavy attenuation during anaphase. The stretch- 

 ing is due to a delayed separation of the daughter 

 chromosomes in a terminally located segment. 

 The formation of sex chromatin in mammals is 

 not an isolated occurrence as similar events 

 cause cell differentiation at embiyogenesis of 

 other animals. Thus, in planarians (Palitdicola, 

 Tricladida) chromosome segments of the blasto- 

 meres become allocyclic, forming chromocenters 

 during interphase, after an attenuation during 

 anaphase. Similarly, more or less long-lasting 

 chromocenters originate in the blastodermic 

 cells of a highei fly {Calliphora erythrocephala). 

 Another result of chromatid attenuation in 

 planarians and Calliphora is diminution of small 

 chromosome fragments, also obviously differ- 

 entiating the somatic cells. 



All these events are initiated by a delayed 

 separation of daughter chromosomes during 

 anaphase of embryonic cells as is also the well 

 known elimination of chromosomes in lower 

 dipterans (Nematocera). 



under the terms of a contract with the 

 Regents of the University of California. 



These observations will be published in full 

 in Hereditas during 1963. 



6.16. The Pattern of DNA Synthesis in the Chromo- 

 somes of the Marsupial Potorous tridactylis 1 . 



Kirsten H. Walen (Berkeley, U.S.A.). 



The marsupial, Potorous tridactylis, has 12 

 chromosomes in the female, and 13 in the male 

 (XY1Y2). The chromosomes are large and all 

 pairs are morphologically distinguishable. Kid- 

 ney and testis cells can be cultured indefinitely 

 and the cell population remains largely diploid. 

 Tritiated thymidine labeling trials have shown 

 the mitotic cycle to be about 32 hr from meta- 

 phase to metaphase. Early sampling of labeled fe- 

 male material showed that silver grains were pre- 

 sent over all the chromosomes except the short arm 

 of one X chromosome. The same procedure with 

 male kidney cells resulted in thesametypeof asyn- 

 chronous uptake of triated thymidine into the 

 single X chromosome. In general both chro- 

 matids were labeled in the first metaphase after 

 exposure to tritiated thymidine, whereas only 

 one chromatid was labeled in second- and third- 

 cycle metaphase figures. Frequently, however, it 

 was observed that the labeling switched from one 

 chromatid to the other. No open chromatid breaks 

 or gross chromosomal aberrations were observed 

 in any of the division cycles. 



6.17. The Ciliate Macronucleus. B. R. Seshachar 

 (Dehli, India). 



The electron microscope has revealed, in the 

 macronucleus of two Spirotrichous ciliates, 

 Blepharisma and Spirostomum, a system of 

 greatly elongated filaments disposed inside it in 

 a random manner. The thickness of the filaments 

 is variable and each individual filament shows 

 varying thickness along its length. Sections of 

 the filament resemble very strikingly those of the 

 chromosomes of higher organisms. They are 

 interpreted as chromosomes of the macronucleus. 

 Each filament is made up of many microfibrils 

 about 150 a thick, also disposed in a random 

 manner but with a slight orientation in the long 

 axis of the filament. Earlier findings of the 

 author show that on centrifugation and cyanide 

 action, the macronucleus of these two ciliates 

 yield long Feulgen positive filaments, corroborat- 

 ing the observations with the electron microscope. 



These greatly elongated chromosomal filaments 

 would have to be interpreted in the light of the 

 special attributes of the ciliate macronucleus: its 

 origin and differentiation from a diploid micro- 

 nuclear product, its large size (believed to be 

 due either to polyploidy or to its compound 

 nature), its capacity for growth and regeneration, 

 its high DNA content, its amitotic division and 

 its genie and metabolic functions. It would also 

 have to be integrated with our knowledge of 

 DNA structure and mitotic chromosome gener- 

 ally. 



Sponsored by the Office of Naval Research 



6.18. Behaviour of Nucleoli in Isolated Nuclei. 



L. F. Lacour (Hertford, Great Britain). 



Nuclei have been isolated from HeLa cells 

 without appreciable loss of nucleic acids (Fisher 

 and Harris, 1962). In isotonic solutions the 

 isolated nuclei swell and the nucleoli disappear 

 but reappear when the nuclei recover normal 

 size in buffered saline. No nucleolar material 

 can be detected in swollen nuclei in the electron 

 microscope (Crawley and Harris, 1963). This 

 behaviour is being investigated by labelling of the 

 nucleolar RNA with 3 H-cytidine before isolation 

 of the nuclei. 



106 



