208 PROTISTS AND DISEASE 



besides the vaccine of Edward Jenner that is used in human 

 medicine. The first injection is an emulsion of infected 

 rabbit's spinal cord desiccated during 14 days ; and, in 

 increasing strength, doses of fixed virus are given for 15 days, 

 the last being of 3-day cord. 



Nucleoli. "The nature of true nucleoli is still imper- 

 fectly known " wrote Wilson in 1904. It may well be that 

 study of the large nucleoli of brain cells in hydrophobia will 

 afford some information. The granules escaping from the 

 nucleus at b in Fig. 66 may represent the immediate means 

 of defence possessed by the brain-cell. Wilson mentions 

 that the subcutaneous gland- cells of Pisciola contain but a 

 single nucleolus, but during growth of the cell the nucleolus 

 fragments into several hundreds of nucleoli, which then 

 appear to migrate into the cytoplasm, leaving but a single 

 one to grow and repeat the process. Writing of chromidia 

 in Part IV I suggested that the specific products of the 

 highly specialised cells of metazoa, for example the secretions 

 of gland-cells, may be chromidial in origin. It seems as 

 though in nuclei that have permanent chromosomes there 

 may be a division of labour, reproduction devolving chiefly 

 on these whilst the nucleolus, in what we call resting nuclei, 

 furnishes secretions of different kinds for diverse purposes. 



Vaccines and pro- vaccines. As stated above there are 

 in human medicine only two vaccines, the Jennerian and the 

 Pasteurian. In general a vaccine is definable as an attenuated 

 culture of a pathogenic organism ; i.e. it contains living 



