FOOD ACCESSORIES AND DIGESTION. 63 



stancy of relative position among the parts of the sarcode, and the rise 

 of a contrast between superficial and central parts, is perhaps best 

 shown in the minutest and simplest Infusoria, the Monadince. The 

 genus Monas is described by Kent as " plastic and unstable in form, 

 possessing no distinct cuticular investment ; . . . the food-substances 

 incepted at all parts of the periphery " * ; and the genus Scytomonas 

 he says " differs from Monas only in its persistent shape and accom- 

 panying greater rigidity of the peripheral or ectoplasmic layer." f De- 

 scribing generally such low and minute forms, some of which have 

 neither nucleus nor vacuole, he remarks that in types somewhat 

 higher "the outer or peripheral border of the protoplasmic mass, 

 while not assuming the character of a distinct cell-wall or so-called 

 cuticle, presents, as compared with the inner substance of that mass, a 

 slightly more solid type of composition." \ And it is added that these 

 forms having so slightly differentiated an exterior " while usually ex- 

 hibiting a more or less characteristic normal outline, can revert at will 

 to a pseud-amoeboid and repent state." # Here, then, we have several 

 indications of the truth that the permanent externality of a certain 

 part of the substance, is followed by transformation of it into a coat- 

 ing unlike the substance it contains. Indefinite and structureless in 

 the simplest of these forms, as instance again the Gregarina, || the 

 limiting membrane becomes, in higher Infusoria, definite and often 

 complex ; showing that the selection of favourable variations has had 

 largely to do with its formation. In such types as the Foraminifera, 

 which, almost structureless internally though they are, secrete calca- 

 reous shells, it is clear that the nature of this outer laver is determined 

 by inherited constitution. But recognition of this consists with the 

 belief that the action of the medium initiated the outer layer, special- 

 ized though it now is ; and that even still, contact with the medium 

 excites secretion of it. 







FOOD ACCESSOEIES AND DIGESTION. 



Br Dr. J. BUENEY YEO. 



MAN, like any other animal, is so much the creature of his food 

 his physical perfection, his intellectual activity, and his moral 

 tone are so dependent on the food he receives and the uses he is able 

 to make of it in the processes of digestion and assimilation that any 

 accurate knowledge, founded on precise and reliable methods of inves- 

 tigation, of the influence on digestion and nutrition of dietetic habits 

 must of necessity be of the most general interest. 



* A Manual of the Infusoria, by W. Saville Kent. Vol. i, p. 232. 



\ lb. Vol. i, p. 241. " % K ent. Vol. i, p. 56. * lb., 1. c. Vol. i, p. 57. 



fl The Elements of Comparative Anatomy, by T. H. Huxley, pp. 7-9. 



