Fig. 78. Diagram of the maturation of a parthenogenetic ovum. The number of chromosomes normal to the species has been assumed to be four. Uei, a primitive germ-cell. M Eiz, a mother-egg-cell, with twice the normal number of chromosomes. Eiz, mature ovum after the separation of the first and only polar body. Rk1.
Among bees the state of affairs is again exceptional. Here the female, the so-called queen bee, possesses a capacious sperm-sac, in which the spermatozoa received in copulation remain living for years, and the fertilization of an ovum is effected in the usual way from this sac while the egg from the ovary is passing down the oviduct. The queen bee has the power of releasing some spermatozoa from the receptacle, or of not doing so, and thus of fertilizing the egg, or of not fertilizing it. Since the notable observations of Dzierzon and the investigations of von Siebold and Leuckart which followed them, it has been assumed that only those eggs were fertilized which were laid in the cells destined for rearing females (workers or queens), while those which were to give rise to 'drones' or males remained normally unfertilized. Only in the last decade of the past century did the bee-keepers begin to cast doubt on this so-called 'Dzierzon theory'; various violent and obstinate attacks were made upon it, and these were supported by new and apparently convincing experiments. Dickel, a teacher in Darmstadt, has been particularly strenuous in attempting to overthrow the old theory, by emphasizing the fact that von Siebold's old investigations on bee eggs afforded no convincing proof. Von Siebold made his investigations on eggs freshly taken from the hive, and was never able to find spermatozoa in 'drone eggs' (that is, eggs laid in drone cells and therefore destined to develop into drones), while he was often able to demonstrate the presence of from one to four spermatozoa in 'worker eggs.' But he only examined 'drone eggs' which were already twelve hours old, and in these, as we now know, he would not have found spermatozoa in any case, even if they had been fertilized, because in ova at that stage the development of the embryo has already fully begun, and nothing remains of the spermatozoa. In the bee, according to Buttel-Reepen, the fertilizing spermatozoon is transformed in twenty minutes after it has penetrated into the egg into a minute 'sperm-nucleus' which is almost invisible even in sections, and certainly nothing whatever could be seen of it by the old method of squeezing the fresh ovum.
It had therefore to be admitted that Dzierzon's theory rested on an insecure foundation, and I accordingly set two of my students at that time, Dr. Paulcke and Dr. Petrunkewitsch, to examine the eggs of the bee anew with regard to the point in question, using the greatly improved methods at their disposal. These investigations have been carried out in the Freiburg Zoological Institute during the last three years, and have resulted in establishing the absolute correctness of Dzierzon's theory: the 'drone eggs' do remain unfertilized, while the eggs from which females are to develop are fertilized without exception.
In this case, therefore, we have, in the same animal, eggs which can be fertilized and eggs which, without fertilization, develop parthenogenetically, and it is therefore of the greatest possible interest to know the state of matters in them in regard to the directive divisions and the reduction of the chromosomes.
Dr. Petrunkewitsch's investigations have shown that in both cases, that is, whether a spermatozoon penetrates into the ovum or does not, a twice-repeated division of the nuclear material in the ovum takes place. Moreover, the two daughter-nuclei which result from the second division do not, as Brauer showed was sometimes the case in Artemia, unite again afterwards; they remain separate, and the number of chromosomes—there are sixteen of them—is thereby reduced to half in the segmentation nucleus. But this is not all, for before embryonic development has begun the normal number can be again seen in the segmentation nucleus; the chromosomes must therefore have doubled their number by division within the nucleus.
It is probable that something similar takes place in the cases of exceptional parthenogenesis which have long been known, but this point has not yet been sufficiently investigated. Nevertheless I cannot pass them over, as they are instructive from another point of view.
Fig. 79. The two maturation divisions of the
'drone eggs' (unfertilized eggs) of the Bee, after
Petrunkewitsch. Rsp 1, the first directive spindle.
k 1 and k 2, the two daughter-nuclei of the same.
Rsp 2, the second directive spindle. k 3 and k 4, the
two daughter-nuclei. In the next stage k 2 and k 3
unite to form the primitive sex-nucleus. Highly
magnified.
In some silk-moths (Bombycidæ) and hawk-moths (Sphingidæ), especially in the silk-moth proper (Bombyx mori), in Liparis dispar, and in quite a number of other Lepidoptera, it sometimes happens that, out of a large number of unfertilized eggs, a few will develop and produce caterpillars. This is interesting enough, but it gains increased importance through the investigations of the Russian naturalist, Tichomiroff, who succeeded in considerably increasing the number of unfertilized eggs that developed by gently rubbing them with a paint-brush, or by dipping them for a little in dilute sulphuric acid. It is thus possible to make eggs, which would not ordinarily develop without being fertilized, capable of parthenogenetic development by means of mechanical or chemical stimulus. This sounds almost incredible, but it is beyond a doubt, and it is still further corroborated by the fact that Prof. Jacques Loeb has succeeded in inciting the eggs of a sea-urchin to parthenogenetic development by means of a chemical stimulus. When he added to the sea-water in which the eggs were laid a certain quantity of chloride of magnesium the ova developed, and not only went through the process of segmentation, but even reached the stage of the quaint easel-like Pluteus larva. Quite recently Hans Winkler has made the interesting observation that from sea-urchin sperms which have been killed by heat it is possible to extract in aqueous solution a substance capable of exciting unfertilized sea-urchin eggs to development, although they only go as far as to the sixteen-cell stage.