It is hardly necessary to point out that in these cases we are seemingly dealing with cases of the inhibition of growth which cannot be explained by the tyranny of the whole over the parts, and that there must be conditions at work other than the mere flow of substances which can cause a cessation of growth. This can be illustrated by certain observations on the egg. 8. The history of the egg shows a reversible condition of rest and of activity. The primordial egg cell multiplies actively until a large number of eggs are formed in the ovary which may reach into the millions in the case of sea urchins or certain annelids. These cell divisions then stop and the egg goes into the resting stage in which it deposits the reserve material for the development of the embryo. From this condition it can only be called into activity again by the spermatozoön or the agencies of artificial parthenogenesis. It seemed of interest to find out whether or not the development of the egg may be reversed once more after it has been activated. From all that has been said in the chapter on artificial parthenogenesis, such a reversal should take place in the cortical layer. The result of these experiments seems to be that if a complete destruction or change in the cortical layer has once taken place--such as that caused by the entrance of a spermatozoön into the egg--no reversal is possible; although the development of the fertilized egg may be suppressed for a long time by either low temperature or lack of oxygen, or, in the case of seeds and spores, by lack of water. But as soon as the conditions for the chemical reactions in the egg are normal again, the development may go on unless the egg has suffered by the methods used to prevent development or by the long duration of the suppression. With an incomplete destruction of the cortical layer both development as well as reversal of development are possible. Thus the writer has shown that in the egg of _Arbacia_ the effect of the cortical alteration of the egg induced by the butyric acid treatment or by the treatment with bases can be reversed. When unfertilized eggs of _Arbacia_ are put for from two to five minutes into 50 c.c. sea water + 2.0 c.c. N/10 butyric acid they will all form a gelatinous, somewhat atypical fertilization membrane; when put back into normal sea water all will perish in a few hours unless they are submitted to the short treatment with a hypertonic solution mentioned in the previous chapter, while if submitted to this treatment they will develop. If, however, these eggs are transferred from the butyric acid sea water not into normal sea water but into sea water containing some NaCN (10 drops of 1/10 per cent. NaCN or KCN in 50 c.c. sea water), and if they remain here for some time (_e. g._ overnight) they will not perish when subsequently transferred back to normal sea water. Such eggs will develop when fertilized with sperm. The activating effect of the membrane formation