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]
[110] McClendon, J. F., Publications of the Carnegie Institution, No. 183, 125; _Am. Jour. Physiol._, 1910, xxvii., 240.
[111] Gray, J., _Proc. Cambridge Philosophical Society_, 1913, xvii., 1.
[112] R. Lillie has recently shown that in a hypotonic solution water diffuses more rapidly into a fertilized than into an unfertilized egg. This is exactly what one should expect since the unfertilized egg is not only surrounded by the cortical layer but also by a thick layer of jelly both of which are lacking in the fertilized egg. It is difficult to understand how this observation can throw any light on the mechanism of development, since water diffuses rapidly enough into the unfertilized egg.
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.