_A Treatise on Comparative Embryology._ By Francis M. Balfour, LL.D., F.R.S. Vol. ii, p. 343 (second edition). Footnote 18: Balfour, l.c. Vol. ii, 400-1. Footnote 19: Balfour, l.c. Vol. ii, p. 401. Footnote 20: For a general delineation of the changes by which the development is effected, see Balfour, l.c. Vol. ii, pp. 401-4. Footnote 21: _See_ Balfour, Vol. i, 149 and Vol. ii, 313-4. NOTE. After the above articles were published, I received from Dr. Downes a copy of a paper “On the Influence of Light on Protoplasm,” written by himself and Mr. T. P. Blunt, M.A., which was communicated to the Royal Society in 1878. It was a continuation of a preceding paper which, referring chiefly to _Bacteria_, contended that— “Light is inimical to, and under favourable conditions may wholly prevent, the development of these organisms.” This supplementary paper goes on to show that the injurious effect of light upon protoplasm results only in presence of oxygen. Taking first a comparatively simple type of molecule which enters into the composition of organic matter, the authors say, after detailing experiments:— “It was evident, therefore, that oxygen was the agent of destruction under the influence of sunlight.” And accounts of experiments upon minute organisms are followed by the sentence— “It seemed, therefore, that in absence of an atmosphere, light failed entirely to produce any effect on such organisms as were able to appear.” They sum up the results of their experiments in the paragraph— “We conclude, therefore, both from analogy and from direct experiment, that the observed action on these organisms is not dependent on light per se, but that the presence of free oxygen is necessary; light and oxygen together accomplishing what neither can do alone: and the inference seems irresistible that the effect produced is a gradual oxidation of the constituent protoplasm of these organisms, and that, in this respect, protoplasm, although living, is not exempt from laws which appear to govern the relations of light and oxygen to forms of matter less highly endowed. A force which is indirectly absolutely essential to life as we know it, and matter in the absence of which life has not yet been proved to exist, here unite for its destruction.” What is the obvious implication? If oxygen in presence of light destroys one of these minutest portions of protoplasm, what will be its effect on a larger portion of protoplasm? It will work an effect on the surface instead of on the whole mass. Not like the minutest mass made inert all through, the larger mass will be made inert only on its outside; and, indeed, the like will happen with the minutest mass if the light or the oxygen is very small in quantity. Hence there will result an envelope of changed matter, inclosing and protecting the unchanged protoplasm—there will result a rudimentary cell-wall.