732 



NAi LjKn 



[NoVEMHEK 17, 192 



and resultiriji in u complete reorganisation of the 

 nuclear app.initus. Jennings has shown that conjuga- 

 tion is not necessarily btncfitial, that the ex-conjugants 

 vary greatly in vitality and reproductive power, and 

 that in most cases the division rate is less than iK'fore 

 conjugation. Woodruff has since May i, 1907, kept 

 under constant conditions in culture a race of Para- 

 miccium. During the sixteen years there have been 

 some ten thousand generations, and there seems no 

 likelihood of or reason for the death of the race so long 

 as proper conditions are maintained. The possibility 

 of conjugation has been precluded by isolation of the 

 products of division in the main line of the culture, 

 and the conclusion is justifiable that conjugation is 

 not necessary for the continued life of the organism. 

 The criticism that Woodrufif's stock might be a non- 

 conjugating race was met by placing the Paramaecia, 

 left over from the direct line of culture, under other 

 conditions, when conjugation was found to occur. 

 Later observations by Erdmann and Woodruff show 

 that a reorganisation of the nuclear apparatus of 

 Paramaecium takes place about every twenty-five to 

 thirty days (forty to fifty generations). This process, 

 termed endomixis (in contrast to amphimixis), seems 

 to be a normal event in the several races of Para- 

 msecium which Erdmann and Woodruff have examined, 

 and it is proved to coincide with the low points or 

 depressions in the rhythm exhibited by Paramaecium. 

 Enriques (1916) maintained a ciliate — Glaucoma 

 Pyrijormis — through 2701 generations without con- 

 jugation, and almost certainly without endomixis. 

 From a single " wild " specimen he raised a large 

 number and found that conjugating pairs were 

 abundant, so that the objection could not be made 

 that this was a non-conjugating race. Enriques then 

 began his culture with one individual, and examined 

 the descendants morning and evening, removing each 

 time a specimen for the succeeding culture. The 

 number of divisions per day varied from nine to 

 thirteen, and as there was no break in the regularity 

 and rapidity of division, and no sort of depression, 

 Enriques concluded that neither endomixis nor con- 

 jugation could have occurred, for these processes take 

 some time and would have reduced considerably the rate 

 of division. These results, especially if they are con- 

 firmed by cytological study of preserved examples, show 

 that for Glaucoma neither conjugation nor endomixis is 

 necessary for continued healthy existence. Hartmann's 

 observations (191 7) on the fia^gellate Eudorina elegans 

 extend the conclusion to another class of Protozoa. 

 He followed this flagellate through 550 generations in 

 two and a half years. The mode of reproduction was 

 purely asexual, and there was no depression and no 

 nuclear reorganisation other than that following fission. 

 The evidence seems sufficient to confirm the view that 

 certain Protozoa, if kept under favourable conditions, 

 can maintain their vigour and divide indefinitely, with- 

 out either amphimixis or endomixis. 



Child (1915) states as the result of his experiments 

 that the rate of metabolism is highest in Paramaecium 

 and other ciliates immediately after fission — " in other 

 words, after fission the animals are physiologically 

 younger than before fission." This view, that re- 

 juvenescence occurs with each fission, derives support 

 from the observations of Enriques and Hartmann, for 



NO. 2820, VOL. I 12] 



no other process was found to Ik* Uii 



the vigour of their organisms in cuitu . 



If, then, fi.ssion is sufficiently frequent — tliai ij», 1: 



conditions for growth remain favoural)le — the i • 



plasm maintains its vigour. If through c) 



the external conditions the division rate : 



rejuvenescence at each fission may not be 



to balance the deterioration taking place bei«vt ,. 



less frequent divisions. Under such conditions ti 



mixis or conjugation may occur with beneficial rc-^i t 



in some cases, but if these processes are precluded t! • n 



is apparently nothing to arrest the ! " - 



or " ageing " observed by Maupas a; 



The culture of tissues outside the Ixxiy is ihrouif _ 

 new light on the conditions requisite for the multipl ■ .1 

 tion and differentiation of cells. R. G. Harrison (kj-: > 

 was the first to devise a successful method by wl : Ij 

 the growth of somatic cells in culture could be follo\, . il 

 under the microscope, and he was able to demonstr.ac 

 the outgrowth of ner\'e-fibres from the central ner\ou.s 

 tissue of the frog. Burrows (1911), after modif' ■": 

 the technique, cultivated nervous ti.ssue. heart ■ 

 and mesenchymatous tissue of the chick in M' 

 plasma and embryonic extract, and this method .; 

 become a well-established means of investigation (•; 

 cell-growth, tissues from the dog, cat, rat, guinea-pit. 

 and man having been successfully grown. One st- - 

 of connective tissue-cells (fibroblasts) from the <i k 

 has been maintained in culture in vigorous condition 

 for more than ten years — that is, for probably som, 

 years longer than would have been the normal V- 

 of life of the cells in the fowl. Heart-cells ma\ ." 

 grown generation after generation — all traces of tht 

 original fragment of tissue having disappeared — thr 

 cells forming a thin, rapidly growing, pulsating sheet 

 Drew (1922) has recently used instead of coagulate" ; 

 plasma a fluid medium containing calcium salts in .; 

 colloidal condition, and has obtained successful grew tii 

 of various tissues from the mouse. He finds t: .st 

 epithelial cells when growing alone remain unfi 

 entiated, but on the addition of connective t; 

 differentiation soon sets in, squamous epitheliun. 

 producing keratin, mammary- epithelium giving ri.>" 

 to acinous branching structures, and when heart -cell 

 grow in proximity to connecti\"e tissue they exhibi- 

 typical myofibrillar, but if the heart-cells grow apar; 

 from the connective tissue they form spindle-shaped 

 cells without myofibrillae. 



For many lines of work in modem zoology bic 

 chemical methods are obviously essential, and thi 

 applications of physics to biology are likewise 1 

 important — e.g. in studies of the form and developi 

 of organisms and of skeletal 'structures. Without 

 entering into the vexed question as to whether all 

 responses to stimuli are capable of explanation in 

 terms of chemistry and phy.sics, it is very evident that 

 modem developments have led to the increasing 

 application of chemical and physical methods tt> 

 biological investigation, and consequently to a closer 

 union between biology, chemistr>', and physics. It 

 is clear also that the association of zoology with 

 medicine is in more than one respect becoming pro- 

 gressively closer. Comparative anatomy and embry- 

 ology, cytology, neurology, genetics, entomology, and 

 parasitology, all have their bearing on human welfare. 



