CYTOLOGY 



3- 



Boveri, however, in 1909 succeeded in demonstrating it, and since then others 

 have augmented his proof, so that the way was open for the recognition of the 

 chromosomes as permanent hereditary units. 



7. Roux in 1883 and Weismann in 1887 had supported the idea that the 

 units of inheritance were arranged in 



a Hnear order in the chromosomes, a 

 view which fitted in with the process 

 of longitudinal splitting in division. 

 This found a modern expression in 

 the theor\' of the " Genes " as the 

 hereditary units in the chromosomes, 

 proposed by Morgan in 1926. Cyto- 

 logical proof of this theory is very 

 recent, but it has been strongly sup- 

 ported by genetical work on the Fruit 

 Fly, Drosophila, which has enabled 

 investigators to identify the positions 

 of many such genes on particular 

 chromosomes. 



8. This brings us to the last and 

 most recent of the basic concepts in 

 cvtology, namely that each chromo- 

 some pair carries a group of associ- 

 ated hereditary characters (linkage 

 groups) and that these may be ex- 

 changed between them by crossing- 

 over during meiosis. 



It is impossible to separate sharply between cytology and genetics at 

 the present time, for the two branches of the science have become closely 

 bound up with one another. In earlier times cytology and genetics travelled 

 along separate paths, and it was not till 1887 when Weismann put forward 

 his theorv of the " Inheritance of the Germ Plasm " that the union of the 

 two branches became established. From that time onwards they have become 

 increasingly united. It follows therefore that it has been impossible in this 

 chapter to separate sharply cytology from genetics, and the student is advised 

 to read this chapter in conjunction with the account of genetics given in 

 Volume IV. 



Fig. 17. 



[Coiir/esy of the Linnean Society. 

 Portrait of Edouard Strasburger. 



SUMMARY OF THE BASIC IDEAS IN CYTOLOGY 



I. Every organism consists of cells. These possess, normally, a cell wall, 

 a colourless semi-liquid material cytoplasm, and a nucleus, both the latter 

 constituting the protoplasm or living substance. In addition the cell may 

 include plastids, which often bear pigments, e.g., chloroplasts. These are 

 embedded in the cytoplasm. The cytoplasm contains immense numbers of 

 minute granules of substances formed by it and also larger units, rod-like 



