THE MITOTIC CYCLE 



a fresh crop of mitoses, renewal of the medium is necessary. Willmer^^'' 

 thinks that : 'in any given colony there are some cells which need high 

 concentrations of juice before they will divide, while others have more 

 modest requirements.' It is perhaps possible to see an analogy between 

 the latent period after application of embryo juice to a culture and the 

 onset of mitosis, and the latent period which intervenes between infec- 

 tion of cells with a virus and the reproduction of the virus (Horsfall;^^^ 

 LuRiA^^^). The analogy with virus reproduction is worth examining. 

 Indeed, Fischer^^ has suggested that the growth-promoting factors 

 'may display biological activity somewhat akin to that of the virus- 

 proteins' and a similar idea had already been put forward in 1936 by 

 DES LiGNERis.i**^ The nucleoprotein-containing particles of embryonic 

 cells have a number of properties which may support the idea that they 

 are concerned in the mitosis-stimulating activity of the extracts. 

 Embryo extracts are essentially cytoplasmic extracts and contain 

 pentose nucleoprotein but (depending on the method of preparation) 

 no detectable deoxypentose nucleic acid (Waymouth, unpublished) or 

 very small amounts (Davidson and Waymouth^^^). From extracts 

 prepared from 9-day embryos by extracting one volume of tissue 

 pulp with one volume of Tyrode solution, 80 per cent of the pentose 

 nucleic acid can be sedimented in one hour at 100,000 X g (Gilbert, 

 Meiklejohn and Waymouth, unpublished). The major part of the 

 pentose nucleoprotein can be spun down from adult tissue extracts at 

 much lower speeds, at which only about 50 per cent of the embryonic 

 particulate material may be sedimentable (Brachet^^^) . Claude^^^"!^^ 

 first isolated particles from 8-day chick embryos, and compared the 

 preparations with the particulate fraction, isolated under similar cen- 

 trifugal conditions (18,000 X g for 90 minutes) with which the Rous 

 sarcoma (chicken tumour I) activity is associated. A particulate fraction 

 from 8-day chick embryos (67,000 X g for 30 minutes) was tested on 

 cultures of mouse fibroblasts in chicken plasma and rat serum (Ten- 

 NANT et alii^^^ 1*''), and the 'expansion rate' {i.e., area increase in 30 

 hours) measured. The activity (in this sense) was not very great and 

 appeared to be increased by heating the pellet material at 100° C. for 

 15 minutes. Fischer^^ reported that high molecular components of 

 embryo extract, separated in the ultracentrifuge, possessed growth- 

 promoting activity, but 'in no way exceeding that of the supernatant 

 fluid', and he does not regard the\activity detected by Tennant et alii^'^'^ 

 as that of a specific growth promoting substance, but rather as due to 

 an 'ordinary nutrient . . . substance'. 



Macromolecular complexes 



During the last decade much new information about the composition 



and functions of the cytoplasm has come to light. It is now clear that 



178 



