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'Generally speaking, Gemmation does not occur indifferently at any point of the body of a Polype, but in the same situations in the same species.

"Some may choose to assert that here we have a transformation of species: e.g. that, on Campanularia geniculata, Medusæ are first formed and then detached, and that these Medusa by retrograde development produce ciliated Infusories.

"The true explanation is obvious: the Medusæ-shaped are the sexual individuals; and the ova of the female, in the earliest embryonal form of Infusories, swim about for the purpose of distributing the species. This embryonal form is little more than a bag of germ-cells. Of these cells a part is employed in the development of the Polype when fixed, and a part retained for the future development of the germs, which are non-sexual, and sexual, in their proper order and position on the stem."

"In many of the Parasitic Worms a still more complicated series of changes prevails in the development of the ovum. In no one species indeed has the whole series of these changes been connected by direct observation. But still enough has been done to establish the remarkable fact, that the embryo after leaving the egg, is not always changed into an individual resembling the parent; but into a new larval being, which produces broods of other larval forms. These last individuals undergo changes which close the series of metamorphoses; and in the end they become perfect animals like their parents.

"To instance in the Trematodes-(such creatures as the Liver Fluke.) In the ovum, after the germinal vesicle has disappeared, several large embryonal cells are observed within the yelk-mass-this last not having undergone the divisional process. It is the embryonal cells themselves which here undergo division and multiplication; the entire mass of them growing and multiplying at the expense of the yelk-mass, and at last occupying its place. The embryo is

now composed of exceedingly minute germ-cells, enclosed in a ciliated epithelium, is of an oval form, and at one end has a suctorial disc. Succeeding stages in the Trematodes have been observed: especially two larval stages, of which one is cylindrical in form, the other caudate. The cylindrical larvæ are more or less perfectly organised; and in their interior new generations of larvæ are formed from original germ-cells, not from ova. This brood of new larvæ, again, may be either cylindrical, or caudate. But it is the caudate form which at length brings the series of metamorphoses to a close-by losing the tail and developing perfect sexual organs, and passing into the form of a complete Trematode.

"When the caudate larvæ have been so far developed as to burst the cylindrical body of the earlier larvæ which enclosed them, they swim about in search of a very different habitat from that of the parent. The Trematode was the parasite of a Mollusc: the caudate larva seeks about for the larva of some Insect. Siebold saw the Cercaria armata, the larva of a Trematode, readily penetrate the larvæ of Ephemera and other insects, which are developed in water, penetrating between the rings. Here it casts its tail, and surrounds itself with a covering, the secretion of its own body, in order to undergo its last change. But Siebold sees reason to doubt that this is completed within the body of the larval insect. Because, though he found numerous Trematode parasites in the bodies of the most different insects, whose larvæ live in water,-as of species of Libellula, Ephemera, Phryganea, &c.-yet he could never discover that their sexual organs were perfectly developed. He suspects that these organs, though they are visible in a rudimentary state in these parasites of insects, can only be developed in another locality; and that the animal obtains this development when it is swallowed, together with the insect it infests, by some bird or other animal.

"Here the original embryonal mass, beginning with the impregnated ovum, passes through a still more remarkable

1st, An

series of changes than those before considered. infusorial form; 2nd, Cylindrical larvæ, worm-shaped, producing a brood from a portion of the unappropriated germcells; 3rd, Caudate larvæ, like Cercaria, the progeny of the former; 4th, Final change from the Cercarial form to that of the perfect creature with sexual organs." (Dr Clark).

In vertebrate animals, the whole spermatic force diffused among the germinal vesicles of the ovum is exhausted in the development of the tissues and organs of the individual. The cycle of fœtal changes is in each case simple, and but one form of larval life is produced. But it does sometimes happen, though very rarely, even in the human species, that a germ-cell has been carried forward, among the animal tissues, with sufficient powers of embryonic development to produce disease and death; but never with sufficient power to bring a fœtal form to maturity.

The essence of the peculiarity in the embryonic development of the lower creatures of the animal kingdom is (on Owen's theory of Parthenogenesis) only this-All the germcells of the ovum are not at once appropriated to the formation of the embryo. A portion of such cells passes on, unchanged from embryo to larva, and from one larval form to another whilst other portions are changed at each stage of advance, and employed in laying down the organs belonging to that period of development: and in this way a complicated cycle is at length completed according to the law assigned to each Species.

"How the retained spermatic force (says Professor Owen) operates in the formation of a new germ-mass from secondary, ternary, or quaternary derivative germ-cells, I

"I have myself (says Dr. Blundell) seen a fœtus, on the whole not imperfectly formed, about the size of six or seven months, and which was taken from the body of a boy where it lay in a sac, in communication with the child's duodenum, the boy being pregnant." (See Lectures on the Principles and Practice of Midwifery, by James Blundell, M. D., London, 1839. p. 481.)

do not profess to explain" (p. 72). Neither is it known. how the same force operates in the development of the primary germ-mass from the first vital germ of the ovum. We witness certain centres of attraction and repulsion, and we find a definite result. We know an anterior cause-the union of a spermatozoon and a germinal vesicle-out of which these attractions and repulsions began. In each species we know also the consequences that follow the first set of organic combinations. The sequence of the phenomena is in each case invariable, and we can therefore properly define it under the terms of an empirical law. But a wise physiologist presumes to go no farther.

The spermatic force seems to be soonest exhausted where the complexity and vital power of the perfect animal is greatest; and to be longest retained in cases where the structure and vital powers are of the lowest order. Thus this force is longest retained, and furthest transmitted, in the vegetable kingdom-the zoophytes manifest it in the next degree-and the power of the retained germ-cells to produce a perfect germ-mass and embryo, by the remnant of spermatic virtue they inherited, is finally lost, so far as is known, in the class of Insects, and in the lower Mollusca. In some creatures, however (such as Crabs and Lobsters, and certain Lizards), which can reproduce portions of their bodies that have been mutilated or broken off, we seem to trace the last fading energy of that transmitted force we have been here describing. In such creatures benevolent nature has deposited a store of unused derivative germ-cells on certain fixed points where they are most likely to be wanted; and it is from such fixed points, and from them alone, that the reproduction of a mutilated organ can begin.

No. V.

On the Development of the Animal and Vegetable Kingdoms in the oldest known Fossiliferous Strata.

(p. lxviii.)

THE best illustration of this point is found in Hall's Paleontology of New York: and I here place before the reader a synopsis of some of the facts brought to light by the American Geologists.

1. Potsdam Sandstone. The very lowest fossil-bearing deposit of North America.

Fucoids (one Genus).

Brachiopoda (Lingula, Orbicula ?)

2. Calciferous Sandstone.

Fucoids (Two Genera).

Crinoidea? an obscure plate.

Brachiopoda (Atrypa, Lingula).

Gasteropoda (Euomphalus, Maclurea, Ophileta,

Turbo, Pleurotomaria).

Cephalopoda (Orthoceras).

3. Chazy Limestone.

Zoophyta (Lam.) Streptoplasma.

Zoophyta (Polyzoa.) Retepora, genus resembling
Gorgonia, Stictopora, Chatetes.
Echinodermata (Actinocrinus, Asterias?)
Crustacea-Illanus, Asaphus, Ceraurus.

Brachiopoda (Leptæna, Orthis, Atrypa, Orbicula).
Gasteropoda (Metoptoma, Maclurea, Scalites,
Pleurotomaria, Capulus, Murchisonia, Raphis-
toma).

Cephalopoda-Bucania, Orthoceras.

4. Bird's-eye Limestone.

Fucoids (One Genus.)

Crustacea-Cytherina ?

Lamellibranchiata-Modiola ?

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