from the part of the plant where it was brought forth. The stem of the rose might have been incrusted with a chain of such connected larvæ as we see the stem of a fucus incrusted with a chain of connected polypes, and only the last developed winged males and oviparous females might have been set free. The connecting medium might even have permitted a common current of nutriment contributed to by each individual to circulate through the whole compound body. But how little of anything essential to the animal would be affected by cutting through this hypothetical connecting and vascular integument and setting each individual free! If we perform this operation on the compound zoophyte, the detached polype may live and continue its gemmiparous reproduction. This is more certainly and constantly the result in detaching one of the monadiform individuals which assists in composing the seeming individual whole called 'Volvox globator'; and so likewise with the leaf-bud. And this liberation Nature has actually performed for us in the case of the Aphis, and she thereby plainly teaches us the true value or signification in morphology of the connecting links that remain to attach together the different gemmiparous individuals of the volvox, the zoophyte, and the plant.

The popular notion of a plant, as we have said, is that it is one individual complex organism, of which the root answers to the nutrient system, the trunk and branches to the skeleton, the leaves to the lungs, and the flowers to the generative organs of an animal: but we never see the stomach converted into a lung, the lungs converted into a testis or ovarium, or reciprocally, as we trace the leaf changed into the stamen, or this by retrograde metamorphosis brought back to the state of a leaf. The stomach of a man if detached would not live and reproduce its kind like the polype of the Sertularia or the monad of the Volvox; nor would the lung manifest its individuality like the leaf of the Bryophyllum under similar circumstances. No!

A flowerless plant is an associated colony of simple organized individuals (phytons) propagating by gemmation.

A flowering plant, with stamens and pistils in the same flower, is not a hermaphrodite individual; but is an organically associated colony of phytons, most of which are simple and gemmiparous, but some through a higher energy have been developed into distinct male and female individuals.

The so-called male of the Diœcious plants is an organically associated colony of gemmiparous phytons with true male individuals; and the so-called female plant is a similar colony of gemmiparous phytons with true or perfect female individuals.

From plants to Aphides inclusive the phænomenon of Parthenogenesis is presented under manifold modifications, alike in this essential that they are all examples of organized beings from the impregnated ovum of which many individuals may be successively developed.

In the Vertebrated and in the higher Invertebrated animals only a single individual is propagated from each impregnated ovum. Organized beings might be divided into those in which the ovum is uniparous, and those in which it is multiparous. This is the first and widest or most general distinction which we have to consider in regard to generation, and in proportion as we may recognize its cause will be our insight into the true condition on which Parthenogenesis depends.

To this end we must revert to what takes place in the impregnated seed or ovum at the beginning of development, and duly ponder over the nature of the first steps in embryonic formation. These, observation has now established to be essentially the same in all organisms. The spermatozoon has been traced to the ovum in the lower Invertebrata and in Insects (where a special organ, the spermatheca, exists to ensure its application), in Mollusks, in Fishes, in Batrachians and up to the mammal. After

impregnation the germinal vesicle disappears and the germyelk (Pl. I. fig. 4, a) contracts. This constant effect of impregnation indicates the presence of an attractive force which has produced a certain condensation of the germ-yelk. If the yelk-membrane (6) were previously in contact with the chorion (c), it has now receded from it, and an albuminous fluid (d) occupies the interspace.

The primary germ-cell (e) soon appears at or near the centre of the germ-yelk. Its nucleus divides (fig. 5), and two germ-cells result; these recede and establish two centres of attraction and assimilative force, around which, in many species, the germ-yelk collects, and is thus divided (fig. 6). Each secondary germ-cell becomes two (fig. 7), and the matter of the germ-yelk is collected around four centres (fig. 8). The same process again takes place in each of these four, producing an eight-fold division, then a sixteen-fold one (fig. 9), and by its rapid repetition the germ-yelk becomes subdivided and assimilated into countless germ-cells (fig. 10), and by their further subdivision the nuclear or granular germ-mass (fig. 11) is established. This is the most common mode in which the primary impregnated germ-cell propagates itself at the expense of the germ-yelk. But in some species the germ-yelk is not attracted round the successively developed centres, but the germ-cells as they are propagated push their way through the germ-yelk and progressively assimilate it: the ultimate result being the same-a germ-mass of countless secondary germ-cells or nuclei. Much extraneous matter has been assimilated and combined with the essential matter of the primary impregnated germ-cell in order to form the material out of which the embryo is to be developed: but something more is done besides increase of bulk by assimilation and division.

The reception of the matter or principle of the spermatozoon by the germinal vesicle is essential to the commencement of this growth by multiplication of cells. When

the primary division of the impregnated germ-cell takes place, it must divide its properties with its matter between the two cells resulting from the spontaneous fission of its nucleus: and this result must follow every subsequent division *. It is scarcely figurative therefore to say that the primary or parent germ-cell has equally divided its spermatic virtue amongst its countless progeny.

What has here been propounded of the influence of the spermatozoon is equally applicable to the pollen-filament, and to those organisms in which the spermatic force has not been concentrated in matter presenting the common form of the spermatozoon or pollen-filament.

In the Polygastria, for example, the condition of spontaneous fission is due to the previous existence of a spermatic nucleus in each moiety which is about to be individualized.

• Professor Kölliker (Muller's Archiv für Physiologie, 1843, p. 137) has summed up the subordinate modifications of this constant and important preliminary process, as follows:

The germ-cells are developed free in the yelk.

The first germ-cells are small

and assimilate the yeik slowly.

Yelk granular.

Bothriocephalus. Ascaris dentata.

Tania. Oxyurus ambiguus.

Distoma tereticolle.

The first germ-cells are large and take up the yelk quickly.

Yelk fluid.
Cucullanus elegans.

The germ-cells clothe themselves with the yelk.

The germ-cells clothe themselves with part of the yelk.

(Partial division.)

Coregonus palaa.

Alytes obstetricans.

Sepia vulgaris.

Loligo sagittata.

The germ-cells clothe themselves with the whole yelk (as in figs. 4-10, Pl. I.).

(Total division.)

Rana. Triton.

Lepus cuniculus.

Canis familiaris.
Ascarides permultæ.
Strongyli nonnulli.
Nereis. Botryllus.
Pycnogonum. Æolidia.

The preliminary.division of that nucleus has been constantly witnessed where the phænomena have been clearly traced. In many of Ehrenberg's beautiful plates the division of the nucleus is shown before that of the entire body of the monad has commenced; and according to the direction of the division of the nucleus, that of the monad has been either longitudinal* or transverset. Two centres of assimilative

* See tab. 36. fig. 7. Chilodon cucullulus, 13, 14, 15.

† Ib. 16, 17, 18. Ehrenberg calls this nucleus of the Polygastria the 'testicle,' and views its division simply in the relation of the necessity of each individual resulting from the general fission having such an organ: meaning that each monad, developed by spontaneous fission, is perfected, as regards its so-called testis, by the spontaneous division of the previous testis, and not by the formation of a new one. But this is not the mode in which the eye, or the circle of teeth, or the pulsating sac, is gained by the second individual from the fission: the division usually takes place so as to include the original organ in one or in the other moiety; and that in which it may be wanting gets the organ by a special and independent development of it. The constancy of the preliminary fission of the nucleus would therefore show that it related rather to the totality of the act itself than to the partial completion of the individual in respect of its being provided with a particular male organ of generation. How then, we may inquire, does the division of the nucleus relate to the performance of the general act of spontaneous fission? Our hope of any insight into this mysterious relationship would be from some light to be derived by analogous phænomena. But with what phænomena is the one in question analogous? Obviously most closely with those which have been observed in the successive fissions of the impregnated germ-cell of those ova, such e.g. as the ova of the Strongylus, best adapted to give a view of the fission analogous to those which the perseverance of Ehrenberg enabled him to trace in the spontaneous fission of the monad.

The same correspondence between the successive generations of the Chlamydomonas and the formation of the germ-mass in the mammalian ovum has been well discerned and illustrated by Dr. Martin Barry, who has observed, "On examining the figures given by Ehrenberg of successive generations of the Chlamydomonas, I see a resemblance to the two, four, eight, &c. groups of cells in the mammiferous ovum too striking, not to suggest that the process of formation must be the same in both; the essential part of this process consisting in the division of the pellucid nucleus." See my 'Lectures on Invertebrata,” p. 24, 1843.

If this preliminary division of the germ-cell to the spontaneous fission

E