3. The action of acids in the mechanism of artificial parthenogenesis provides some interesting physio­logical problems. When unfertilized sea-urchin eggs are left in sea water containing any of the lower fatty acids up to capronic, the eggs will form no membranes, while in such sea water, and they will show no outer signs of cytolysis (swelling). When, however, the eggs are left in sea water containing any of the fatty acids from heptylic upward the eggs will form membranes while in the acid sea water and soon afterward will cytolyze completely and swell enormously. In solu­tions of the mineral acids no membranes are formed and none are formed as a rule when the eggs are transferred back to sea water. When both a mineral and a lower fatty acid, e. g., butyric, are added to sea water the mineral acid acts as if it were not present, i. e., the eggs form membranes when transferred back to sea water if the concentra­tion of the butyric acid is high enough. All these data are comprehensible if we assume that only that part of the acid causes membrane forma­tion which is lipoid soluble, while the water soluble part is not involved in the process of membrane forma­tion; and that the cytolysis or swelling of the whole egg can only take place in the higher fatty acids (heptylic or above) which are little soluble in water and very soluble in lipoids, while the lower fatty acids, whose water solubility is comparatively high, can only bring about a cytolysis and swelling in the cortical layer but not in the rest of the egg. This makes it appear as though the part undergoing an altera­tion in membrane forma­tion was a lipoid; and this would harmonize with the assump­tion that the specific membrane-inducing substance in the spermato­zoön is not soluble in water, but soluble in fat.

4. These and other observa­tions led the writer to the view that the essential process which causes development might be an altera­tion of the surface of the egg, in all probability an altera­tion of the superficial layer probably of the nature of a superficial cytolysis. The ques­tion remains: What could be the physico­chemical nature of this cytolysis? The writer had suggested in former papers that in the cytolysis underlying membrane forma­tion lipoids were dissolved, and he supposed that the substance to be dissolved might be a calcium-lipoid compound which might form a continuous layer under the surface of the egg.[90] v. Knaffl, working on the cytolysis of eggs in the writer’s laboratory, gave the following idea of the process:

Protoplasm is rich in lipoids; probably it is mainly an emulsion of these and proteins. Any physical or chemical stimulus which can liquefy the lipoids causes cytolysis of the egg. The protein of the egg can really only swell or be dissolved if the condi­tion of aggrega­tion of the lipoid is altered by chemical or physical agencies. The mechanism of cytolysis consists in the liquefac­tion of the lipoids and thereupon the lipoid-free protein swells or is dissolved by taking up water. . . . Hence this supports Loeb’s view that membrane forma­tion is induced by the liquefac­tion of lipoids.[91]

The writer suggested that the destruc­tion of an emulsion in the cortical layer might possibly be the essential feature of the altera­tion leading to membrane forma­tion and development. It had been long observed that unfertilized starfish eggs may begin to develop apparently without any outside “stimulus,” and A. P. Mathews found that slight mechanical agita­tion of these eggs in sea water increased the number which developed. It has been shown in numerous experi­ments by Delage, R. S. Lillie, and the writer, that the substances causing development in the starfish egg are identical or closely related to those which bring about this effect in the egg of the sea urchin and in both cases the development is preceded by a membrane forma­tion.

But how can membrane forma­tion be produced by mere agita­tion? It seems to me that this can be understood if we suppose that it depends upon the destruc­tion of an emulsion in the cortical layer of the egg. It is conceivable that in the egg of certain forms the stability of this emulsion is so small that mere shaking would be enough to destroy it and thus induce membrane forma­tion and development.[92]

The durability of emulsions varies, and where an emulsion is very durable shaking has no effect, while where it is at the critical point of separating into two continuous phases a slight shaking will bring about the separa­tion, and where the emulsion is still less durable we observe the phenomenon of a “spontaneous” parthenogenesis. Eggs like those of most sea urchins belong to the former, eggs like those of some starfish and annelids belong to the second or third type.

It is impossible to state at present whether the fertiliza­tion membrane is preformed in the fertilized egg and merely lifted off from the egg or whether its forma­tion is due to the hardening of a colloidal substance separated from the emulsion (or excreted) and hardened in touch with sea water. But we can be sure of one thing, namely, that the liquid between egg and membrane contains some colloidal substance which determines the tension and spherical shape of the membrane. The membrane is obviously permeable not only to water but also to dissolved crystalloids, while it is impermeable to colloids. When we add some colloidal solu­tion (e. g., white of egg, blood serum, or tannic acid) to the sea water containing fertilized eggs of purpuratus, the membrane collapses and lies close around the egg; while if the eggs are put back into sea water or a sugar solu­tion the membrane soon assumes its spherical shape. This is intelligible on the assump­tion that in the process of membrane forma­tion (or in the destruc­tion of the emulsion in the cortical layer) a colloidal substance goes into solu­tion which cannot diffuse into the sea water since the membrane is impermeable to the colloidal particles. The membrane is, however, permeable to the constituents of sea water or to sugar. Consequently sea water will diffuse into the space between membrane and egg until the tension of the membrane equals the osmotic pressure of the colloid dissolved in the space between egg and the membrane. If we add enough colloid to the outside solu­tion so that its osmotic pressure is higher than that of the colloidal solu­tion inside the membrane the latter will collapse.

It should also be stated that the unfertilized eggs of many marine animals are surrounded by a jelly (chorion) which is dissolved when the egg is fertilized.[93] The writer has shown that the same chemical substances which will induce membrane forma­tion and artificial parthenogenesis will as a rule also cause a swelling and liquefac­tion of the chorion.

We have devoted so much space to the mechanism of membrane forma­tion since it is likely to give a clearer insight into the physico­chemical nature of physio­logical processes than the phenomena of muscular stimula­tion and contrac­tion or nerve stimula­tion, upon which the majority of physiologists base their conclusions concerning the mechanism of life phenomena.

Before we come to the discussion of the second factor in the activa­tion of the egg it should be stated more definitely that for the eggs of some forms the first factor, the process underlying membrane forma­tion, suffices for the development of the egg into a larva and that no second factor is required in these cases. This is true for the eggs of starfish and certain annelids. Thus in 1901 Loeb[94] and Neilson showed that a short treatment with HCl and HNO3 sufficed to cause some eggs of Asterias in Woods Hole to develop into larvæ without a second treatment being needed, and Delage[95] showed the same for CO2; and in 1905 the writer found that the eggs of the Californian starfish Asterina can be induced to form a membrane by butyric acid treatment and that ten per cent. of these eggs developed into normal larvæ. Quite recently R. S. Lillie observed that the eggs of Asterias at Woods Hole can be caused to form membranes and develop into larvæ by a treatment with butyric acid and that the time of exposure required to get a maximal number of larvæ varies approximately inversely with the concentra­tion of the acid, within a range of 0.0005 to 0.006 N butyric acid. If the exposure is too short membrane forma­tion will occur without normal development.[96]