D Parthenogenesis. "Parthenogenesis" is the term employed to designate certain singular phenomena, resulting in the production of new individuals by virgin females without the intervention of a male. By Professor Owen, who first employed the term, parthenogenesis is applied also to the processes of gemmation and fission, as exhibited in sexless beings or in virgin females; but it seems best to consider these phenomena separately. Strictly the term parthenogenesis ought to be confined to the production of new individuals from virgin females by means of ova, which are enabled to develop themselves without the contact of the male element. The difficulty in this definition is found in framing an exact definition of an ovum, such as will distinguish it from an internal gemma or bud. No body, however, should be called an "ovum" which does not exhibit a germinal vesicle and germinal spot, and which does not exhibit the phenomenon known as segmentation of the yelk. Moreover, ova are almost invariably produced by a special organ, or ovary.
As examples of parthenogenesis, we may take the cases of the Plant-lice (Aphides), the Honey-bee, and certain Crustacea; though in the case of the first of these it is possible that the phenomena observed may admit of explanation otherwise than as an instance of parthenogenesis strictly so called.
The Aphides, or plant-lice, which are so commonly found parasitic upon plants, are seen towards the close of autumn to consist of male and female individuals. By the sexual union of these true ova are produced, which remain dormant through the winter. At the approach of spring these ova are hatched; but instead of giving birth to a number of males and females, all the young are of one kind, variously regarded as neuters, virgin females, or hermaphrodites. Whatever their true nature may be, these individuals produce, viviparously, a brood of young which resemble themselves; and this second generation, in like manner, produces a third, - and so the process may be repeated, for as many as ten or more generations, throughout the summer. When the autumn comes on, however, the viviparous Aphides produce - in exactly the same manner - a final brood, but this, instead of being composed entirely of similar individuals, is made up of males and females. Sexual union now takes place, and ova are produced and fecundated in the ordinary manner.
The viviparous Aphides are either wingless or winged; and the number of young produced is so great, that it has been calculated that a single Aphis might in this way be, during the summer months, and by the time the tenth generation was reached, the progenitor of no less than one quintillion of individuals. Each viviparous Aphis possesses an ovary, which only differs from that of the fertile females in being without certain secondary adjuncts (the colleterial gland and sperma-theca). This "pseudovarium" produces egg-like bodies or "pseudova," which are directly developed into young Aphides - the latter being thus produced by "the individualisation of previously organised tissue."
The differences between the "pseudova" and true ova are in no way anatomical, but are wholly physiological; and the decision involved in the viviparous reproduction of the Aphides turns simply upon the question as to whether the viviparous individuals possess, in addition to the pseudovarium, a testis, or whether male organs are absent. Most observers maintain that the viviparous Aphides are wholly destitute of male organs of reproduction, in which case the phenomena just described can only be explained as an example of parthenogenesis. On the other hand, Balbiani maintains that the viviparous Aphides are really hermaphrodite, in which case, of course, the phenomena are of a much less abnormal character.
In the second case of alleged parthenogenesis which we are about to examine - namely, in the honey-bee - the phenomena which have been described are now generally accepted as free from doubt. A hive of bees consists of three classes of individuals: 1, a "queen," or fertile female; 2, the "workers," which form the bulk of the community, and are really undeveloped or sterile females; and, 3, the "drones," or males, which are only produced at certain times of the year. We have here three distinct sets of beings, all of which proceed from a single fertile individual; and the question arises, In what manner are the differences between these produced ? At a certain period of the year the queen leaves the hive, accompanied by the drones (or males), and takes what is known as her "nuptial flight" through the air. In this flight she is impregnated by the males ; and in virtue of this single impregnation, she is enabled to produce fresh individuals for a lengthened period, the semen of the males being stored up in a receptacle which communicates by a tube with the oviduct, from which it can be shut off at will. The ova which are to produce workers (undeveloped females) and queens (fertile females) are fertilised on their passage through the oviduct, the semen being allowed to escape into the oviduct for this purpose. The subsequent development of these fecundated ova into workers or queens depends entirely upon the form of the cell into which the ovum is placed, and upon the nature of the food which is supplied to the larva. So far there is no doubt as to the nature of the phenomena which are observed. It is asserted, however, by Dzierzon and Siebold, that the males or drones are produced by the queen from ova which she does not allow to come into contact with the semen as they pass throught the oviduct. This assertion is supported by the fact that if the communication between the receptacle for the semen and the oviduct be cut off, the queen will produce nothing but males. Also, in crosses between the common honey-bee and the Ligurian bee, the queens and workers alone exhibit any intermediate characters between the two forms, the drones presenting the unmixed characters of the queen by whom they were produced.
If these observations are to be accepted as established - and there can be no hesitation in accepting them as in the main correct - then the drones are produced by a true process of parthenogenesis; but some observers maintain that the development of any given ovum into a drone is really due - as in the case of the queens and workers - to the special circumstances under which the larva is brought up.*
* In the case of Polistes Gallica, Von Siebold appears to have proved.
Among the Crustaceans, parthenogenesis has been established as occurring in some of the water-fleas (Cladocera) and in various of the Phyllopods (Apus, Limnadia, Artemia, etc.) In these latter it is the female which is produced parthenogen-etically; whereas in the honey-bee and in Polistes it is the male.
There are various other cases in which parthenogenesis is said to occur, but the above will suffice to indicate the general character of the phenomena in question. The theories of parthenogenesis appear to be too complex to be introduced here; and there is the less to regret in their omission, as naturalists have not yet definitely adopted any one explanation of the phenomena to the exclusion of the rest.
From the phenomena of asexual reproduction in all its forms, M. de Quatrefages has deduced the following generalisation :
"The formation of new individuals may take place, in some instances, by gemmation from, or division of, the parent-being; but this process is an exhaustive one, and cannot be carried out indefinitely. When, therefore, it is necessary to insure the continuance of the species, the sexes must present themselves, and the germ and sperm must be allowed to come in contact with one another."
It should be added that the act of sexual reproduction, though it insures the perpetuation of the species, is very destructive to the life of the individual. The formation of the essential elements of reproduction appears to be one of the highest physiological acts of which the organism is capable, and it is attended with a corresponding strain upon the vital energies. In no case is this more strikingly exhibited than in the case of insects, many of which pass the greater portion of their existence in a sexually immature condition, and die almost immediately after they have become sexually perfect and have consummated the act whereby the perpetuation of the species is secured.