54 



INTRODUCTION TO CYTOLOGY 



Opposed to the general validity of the foregoing interpretation are 

 the results obtained by a number of investigators^^ who have seen threads 

 or a reticulum in apparently healthy nuclei under carefully controlled 

 conditions of observation (Fig. 23). Martens, for example, working with 

 the plumose stigmas of Arrhenatherum, stamen hairs of Tradescantia, and 

 certain other tissues, watched the action of fixing fluids upon living nuclei 



and found that the reticulate structure visible in 

 healthy nuclei before fixation remained unchanged, 

 except for a slight swelling of the threads and 

 increased visibility due to alterations in refractive 

 index. The same details appeared in material fixed 

 for longer periods and stained with iron alum- 

 hsematoxylin. These results, taken together with 

 analogous findings of Chambers (1924, 1925) on the 

 prophases in animal spermatocytes, indicate that the 

 reticulum appearing in a well-fixed nucleus fairly 

 represents a delicate thready structure actually 

 present during life and rendered more distinctly 

 visible by fixation. It is doubtless true that no fixer 

 gives a wholly perfect rendition of all details of the 

 living structure and that many fluids produce very 

 great alterations, but it now seems evident that the 

 reticulum cannot be explained away as merely a crea- 

 ^g^ tion of technical procedures. 



The reticulum is composed of threads. On the 



crooked chromonem- ^ • i- • ^ j_ ^ x i • r^\ j. -v -j. 



ata. {Photograph ftir- basis of evidence to be presented m Chapter X, it 

 nished by H. Teiezyn- may be Said that thcsc threads occur in a charac- 

 teristic number and are derived each from the 

 filamentous constituent (chromonema) of a chromosome. They appear to 

 be connected laterally by fine strands {anastomoses) about whose origin 

 and nature very little can be confidently affirmed (see p. 135). Since the 

 main threads tend to be crooked and not appreciably thicker than their 

 connecting strands, it is difficult or impossible to recognize them as indi- 

 viduals in the fully developed reticulum. They are characteristically 

 chromatic: they usually stain well, particularly with certain basic dyes 

 (basichromatism) and so offer a strong contrast with the achromatic 

 karyolymph. In some nuclei the threads are rather evenly basichromatic 

 throughout, the reticulum appearing as a system of uniform strands with 

 only slight swellings here and there, especially at the nodes. In other 

 cases the chromaticity may be more pronounced in the swellings, the 

 other regions of the threads being relatively colorless or even oxychro- 



i^Lundegardh (1912a), Kite (1913), de Litardiere (19216), Martens (1924, 1927a6c, 

 19296), Scarth (1927), Belaf (19296, 1930), Guilliermond (1930o), Telezynski (1930), 

 Belling (1933). 



Fig. 23. — Living nu 

 cleus in Tradescantia 

 stamen hair moun 

 in paraffin oil. Note 



