Section 6 — Cytology 



6.39. Nucleic Acid Profiles of Germ Cells of the 

 Mouse. John H. D. Bryan (Ames, U.S.A.). 



Primordial germ cells may be distinguished 

 both from their definitive counterparts in the 

 adult, and from adjacent somatic cells by their 

 large nuclei and chromosomes. The present work 

 forms part of an attempt to determine the proba- 

 ble chemical basis underlying these size diffe- 

 rences. The results of these experiments should 

 also contribute to our understanding of chro- 

 mosome structure. The distribution of DNA and 

 RNA in sections of acetic-alcohol fixed fetal and 

 adult gonads and liver tissue has been studied 

 following simultaneous visualization by means 

 of azure B staining^). 



In certain cases staining was carried out follow- 

 ing treatment with enzymes (trypsin and/or 

 RNAase), or chemical extraction of RNA (cold 

 10 per cent perchloric acid). The results obtained, 

 though qualitative in nature, suggest that there 

 may be differences between the nucleic acid- 

 protein complexes of primordial germ cells and 

 their definitive counterparts. Somatic cells, on the 

 other hand, appear to react to the staining experi- 

 ments in a manner essentially identical with that 

 of the definitive germ cells. 



Supported by grant CA 05591-02 from the 

 National Cancer Institute, U.S.P.H.S. 



1. Flax and Himes, Physiol. Zool. 25, 297-311, 

 1952. 



6.40. (D.) Endomitosis during Megasporogenesis in 

 Saccharuni. G. Bremer (Wageningen, The 

 Netherlands). 



In many interspecific hybrids of this genus it 

 was found by Bremer in 1920, and afterwards 

 also by other investigators, that the somatic 

 chromosome number of such hybrids equals the 

 sum of twice the haploid number of the female 

 parent plus once the haploid number of the male 

 parent. It appeared to Bremer that this pheno- 

 menon is based on endo-duplication of chromo- 

 somes during megasporogenesis after a normal 

 first meiotic division reducing the chromosome 

 number to the half. 



The endo-duplication will be shown in micro- 

 scopical slides with drawings to match. 



6.41 . Sites of Formation of the Extra Nucleoli during 

 Early Oocyte Growth in the Freshwater Teleost 

 Salvelinus fontinalis Mitchill. L. A. Choui- 

 nard (Quebec City, Canada). 



The behaviour of nuclear structures during 



early oocyte growth in Salvelinus fontinalis will 

 be described with special attention focused on 

 the sites at which the extra nucleoli first appear. 

 In the species investigated, the process of extra 

 nucleoli formation begins at about the mid- 

 pachytene stage of meiotic prophase and pro- 

 ceeds in an uninterrupted fashion until the late 

 strepsitene stage is reached; during that period 

 of time, close to a hundred extra nucleoli form 

 within the nuclear cavity. From the mid-pachy- 

 tene up to about the mid-strepsitene stage, our 

 observations suggest that, while some of the 

 extra nucleoli arise by successive growth and 

 detachment from a single large heterochromatic 

 body adjacent to the oocyte's original nucleolus, 

 the others develop in contact with minute chro- 

 mocenters located in the peripheral portion of 

 the nuclear cavity. The relevant observational 

 evidence would seem to favour the view that the 

 heterochromatic body adjacent to the oocyte's 

 original nucleolus contains one of the four 

 alleged pairs of nucleolar organizing regions of 

 the usual nucleolar chromosomes. During the 

 mid-strepsitene stage, on the other hand, our 

 observations reveal that, while the heterochro- 

 matic body and the chromocenters disappear 

 from view, additional extra nucleoli develop at 

 various points along the length of a number of 

 chromosomes. These observations are taken to 

 indicate that non-heterochromatic chromosomal 

 regions distinct from the nucleolar organizing 

 regions of the usual nucleolar chromosomes are, 

 during that period of meiotic prophase, instru- 

 mental in the formation of extra nucleoli. The 

 significance of the above findings will be dis- 

 cussed in relation to the observations of othei 

 workers on the sites of formation of nucleoli in 

 general. 



6.42. An Investigation of DNA Replication in Bovine 

 Sex Chromosomes. Stanley M. Gartler and 

 Barbara E. Burt (Seattle, U.S.A.). 



Both the X and Y chromosomes in cattle can 

 be unambiguously distinguished from the auto- 

 somes. The X is a large sub-metacentric chromo- 

 some, the Y a small metacentric and all the 

 autosomes are acrocentric. DNA replication 

 studies using tritiated thymidine in cell cultures 

 have shown the following: in the male the Y is 

 the last chromosome in the complement to 

 replicate while the short arm of the X replicates 

 shortly before it. The long arm of the X replicates 

 at a distinctly earlier period. In the female one 

 X replicates as in the male while the other X 



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