Another possibility is that the act of fertiliza­tion increases the permeability of the egg. This idea, which seems attractive, was first suggested and discussed by the writer in 1906.[107] He had found that when fertilized and unfertilized eggs were put into abnormal salt solu­tions, e. g., pure solu­tions of NaCl, the fertilized eggs died more rapidly than the unfertilized eggs and he pointed out that these experi­ments suggested the possibility that fertiliza­tion increases the permeability of the egg for salts. The reason for his hesita­tion to accept this interpreta­tion was, that the fertilized egg is also more easily injured by lack of oxygen than the unfertilized egg and in this case the greater sensitiveness of the fertilized egg was obviously due to its greater rate of metabolism. Later experi­ments by the writer showed that the fertilized egg can be made more resistant to abnormal salt solu­tions if its development is suppressed by lack of oxygen or by KCN or by certain narcotics. With our present knowledge it does not seem very probable that lack of oxygen diminishes the permeability of the egg, but we know that it inhibits the developmental processes. Warburg has made it appear very probable that the fertilized egg is impermeable for NaOH and if this is the case it should also be impermeable for NaCl.[108]

The idea that fertiliza­tion and membrane forma­tion cause an increase in the permeability of the egg was later accepted and elaborated by R. Lillie. This author assumes that the unfertilized egg cannot develop because it contains too much CO2 but that the CO2 can escape from the egg as soon as its permeability is increased through the destruc­tion of the cortical layer of the egg.[109] After the CO2 has escaped, the excessive permeability must be restored to its normal value and this is the rôle of the hypertonic treatment. It is, however, difficult to harmonize the assump­tion of an impermeability of the unfertilized egg for CO2 with the fact that if the unfertilized sea-urchin egg is cut into two, as is done in merogony, no development takes place, while such pieces will develop when a spermato­zoön enters. The cortical layer is removed along the cut surface and there is no reason why the CO2 should not escape. Besides, the experi­ments of Godlewski and the writer prove that the cortical layer of the unfertilized sea-urchin egg is apparently very permeable for CO2 since the latter causes membrane forma­tion if contained in the sea water in sufficiently high concentra­tion.

Lillie assumes that the hypertonic treatment restores the permeability raised to excess by the butyric acid treatment, but this assump­tion is not in harmony with the following facts. The writer has shown that it is immaterial whether the eggs are treated first with the hypertonic solu­tion and then with butyric acid or the reverse, if only the eggs remain longer in the hypertonic solu­tion when the hypertonic treatment precedes the butyric acid treatment. It was stated in the beginning of this chapter that the development of the egg can be induced by hypertonic sea water, and we know the reason since hypertonic sea water is a cytolytic agency. The writer found that when we expose unfertilized eggs of purpuratus for from two to two and a half-hours to hypertonic sea water they will often not develop and only a few eggs will undergo the first cell divisions, then going into a condi­tion of rest. When these eggs, both the segmented and unsegmented, were treated twenty-four or thirty-six hours later with butyric acid, so that they formed a membrane, they all developed into larvæ without further treatment. It is impossible to apply Lillie’s theory to these facts, for the simple reason that the treatment with hypertonic sea water was just long enough to induce development in some eggs and hence according to Lillie’s ideas must have increased the permeability of these eggs. Yet these same eggs were induced to develop normally when subsequently treated with butyric acid, which according to Lillie also acts by increasing the permeability. Nothing indicates that the treatment of the eggs with a hypertonic solu­tion diminishes their permeability; the reverse would be much more probable.

Lillie’s theory also fails to explain that mere treatment of the eggs with a hypertonic solu­tion can bring about their development into larvæ. This, however, is intelligible on the assump­tion that the hypertonic solu­tion in this case has two different effects, first a cytolysis of the cortical layer of the egg and second an entirely different effect, possibly upon the interior of the egg, which represents the second or corrective effect.

McClendon[110] has shown that the electrical conductivity of the egg is increased after fertiliza­tion, and J. Gray[111] has found that this increase in conductivity is only transitory and disappears in fifteen minutes. This might indicate that the egg becomes transitorily more permeable for salts after the entrance of the spermato­zoön or after membrane forma­tion; although an increase in conductivity might be caused by other changes than a mere increase in permeability of the egg. The writer is of the opinion that it is necessary to meet all these and other difficulties before we can state that the altera­tion of the cortical layer, which is the essential feature of development, acts chiefly or exclusively by an increase in the permeability of the egg.[112]

7. When the experi­ments on artificial parthenogenesis were first published they aroused a good deal of antagonism not only among reac­tionaries in general but also among a certain group of biologists. O. Hertwig had defined fertiliza­tion as consisting in the fusion of two nuclei, the egg nucleus and the sperm nucleus. No such fusion of two nuclei takes place in artificial parthenogenesis since no spermato­zoön enters the egg, and it became necessary, therefore, to abandon Hertwig’s defini­tion as wrong. The objec­tion raised that the phenomena are limited to a few species soon became untenable since it has been possible to produce artificial parthenogenesis in the egg of plants (Fucus, according to Overton) as well as of animals, from echinoderms up to the frog; and it may possibly one day be accomplished also in warm-blooded animals. A second objec­tion was that the eggs caused to develop by the methods of artificial parthenogenesis could never reach the adult stage and that hence the phenomenon was merely pathological. There was no basis for such a statement, except that it is extremely difficult to raise marine invertebrates. Delage[113] was courageous enough to make an attempt to raise partheno­genetic larvæ of the sea urchin beyond the larval stage and he succeeded in one case in carrying the animal to the mature stage. It proved to be a male.

Better opportunities were offered when a method was discovered which induced the development of the unfertilized eggs of the frog. In 1907, Guyer made the surprising observa­tion that if he injected lymph or blood into the unfertilized eggs of frogs he succeeded in starting development and he even obtained two free-swimming tadpoles. “Apparently the white rather than the red corpuscles are the stimulating agents which bring about development, because injec­tions of lymph which contains only white corpuscles produce the same effects as injec­tions of blood.” Curiously enough, Guyer thought that probably the cells which he introduced and not the egg were developing. In 1910, Bataillon showed that a mere puncture of the egg with a needle could induce development but he believes that for the full development the introduc­tion of a fragment of a leucocyte is required. Bataillon has called atten­tion to the analogy with the writer’s results on lower forms, the puncturing of the egg corresponding to the cytolysis of the surface layer of the egg and the introduc­tion of a leucocyte as the analogue of the second or corrective factor. The method of producing artificial parthenogenesis by puncturing the egg has thus far been successful only in the egg of the frog. The writer has tried it in vain on the eggs of many other forms. He has at present seven partheno­genetic frogs over a year old, produced by merely puncturing the eggs with a fine needle (Fig. 6). These frogs have reached over half the size of the adult frog. They can in no way be distinguished from the frogs produced by fertiliza­tion with a spermato­zoön. This makes the proof conclusive that the methods of artificial parthenogenesis can result in the produc­tion of normal organisms which can reach the adult stage.