ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 209 



functional cytoplasm is normally large. The thyroid is only slightly 

 differentiated, compared with many other organs, and its retrogression is 

 therefore accomplished without striking transformations. 



Structure of Dig^ital Tendons.* — F. de Fenis points out that each 

 long flexor tendon of the digits in man and mammals has the structure 

 of two juxtaposed strings the fibres of which are twisted in opposite 

 directions. This has the effect of uniformly distributing the effort of 

 traction to all the fibres of the tendon. It is more marked in proportion 

 to the activity of the tendon. The author discusses the mechanical 

 factors in the evolution of this adaptation. 



So-called Intestinal Glands in Necturus.f — H. T. Mead discusses 

 the nature of certain protruding groups of cells embedded in the 

 submucosa and connected to the mucosa. Groups of similar cells have 

 been described in newts, salamanders and the like. As there is no 

 lumen the protuberances cannot be glands. Cells in various stages of 

 mitosis are frequent in all parts of the protuberance, but not in the 

 mucosa at any considerable distance from a protuberance. It seems that 

 the cells which are to compose the intestinal mucosa in Necturus are 

 formed in these protuberances. They are centres for cell-proliferation, 

 as has been concluded for some other Urodela by Bizzozero and 

 Nicholas. 



C- General. 



Ag^e-cycles and Periodicities in Organisms. | — C. M. Child com- 

 bats the idea that ageing is irreversible. The fact is that many cells,, 

 even in the higher animals, may undergo more or less de-differentiation 

 as well as differentiation. Life is a cycle of alternating periods of 

 senescence and rejuvenescence. The general metabolic rate in the 

 simple animals, particularly certain Planarian worms, decreases from a 

 very early stage in development ; the rate of growth also decreases ; in 

 short, the animals undergo senescence. But a piece cut off undergoes 

 more or less de-differentiation and internal re-organization, and usually 

 develops into a new whole. It uses up part of its own body-substance 

 as a source of energy and of the new cell material formed. The new 

 individual is physiologically younger than the original animal, and the 

 smaller the piece was the younger the new individual is. The new 

 individual will grow old, and may be cut into pieces again. These will 

 show rejuvenescence, and this alternation has been kept up experi- 

 mentally for twenty generations. The process of reconstitution somehow 

 brings about rejuvenescence. 



Starved Planarians become gradually smaller. During the reduc- 

 tion the metabolic rate increases ; they become younger. By feeding it 

 is possible to stop the process of rejuvenescence. One species of 



* Arch. Zool. Exp6r., Iv. (1915) pp. 19-45 (22 figs.). 



t Trans. Amer. Micr. Soc, xxxv. (1916) pp. 125-30 (1 fig.). 



X Proc. Amer. Phil. Soc, Iv. (1916) pp. 330-9. 



