t an agency of another sort is not out of the question. It is conceivable that a chemical stimulant, elaborated by the neoplastic cells, might cause the tumour in another host and bring about in consequence a further production of the same stimulant". The tumour inducing filtrates became known as "filterable viruses" or oncoviruses and, more recently, exogenous retroviruses and infectious retroviruses.(17) In the 1950s, in animal cultures and in fresh tissue, especially tumour tissue, particles later attributed to retroviruses, were readily detectable with electron-microscopy (EM). In 1970, the enzyme reverse transcriptase (RT), which transcribes RNA into DNA, was discovered in oncoviruses. Because of this, in the 1970's, oncoviruses became known as retroviruses. In the preceding decade, density gradient centrifugation was introduced to separate and isolate sub-cellular particles including viruses. Because some cellular constituents were found to have the same buoyant density as viruses, when viruses were isolated from cell cultures, the best results could be obtained with supernatant fluids which had high viral concentration and low cellular contaminants. This was best satisfied by non-cytopathic viruses and by culture conditions which maintained maximum cellular viability. All retroviruses isolated prior to HIV satisfy the above conditions.(19) Taking advantage of the above retroviral properties, by repeated suspensions and sedimentation in sucrose density gradients, one could obtain, at a density of 1.16 gm/ml, a relatively pure concentration of retroviral particles-that is, obtain retroviral particles separate from everything else, and thus isolate them.(19) Nonetheless, as many eminent retrovirologists point out, contamination of the viral preparation with particles which contain RT, but could be nothing more than "cellular fragments", microsomes from disrupted cells, "membraneous vesicles which may enclose other cellular constitue...
