one common end,-the multiplication of the race, though by different means. The great function of the germinal or protomorphic zooids is the evolution of the more perfect embryos, of which they serve as budding-stocks; that of the sexual or gamomorphic zooids is the development of ova and spermatozoa. These ends accomplished, their vitality ceases, while the typical organism, the offspring of the former class, or the parent stock of the latter, as the case may be, has a much more permanent duration, and may go on for a long time in perfect vigour, sending off crop after crop of ova, or of sexual gemmæ, according to its mode of propagation.

The distinctness of these varieties of alternation is further shown by their occasional co-existence in the same species, as in some Cestoid worms, and perhaps in a more latent form in the case of the Polyzoa* and of some Annelida. These, however, are exceptional cases, for it would appear that organisms which are propagated by protomorphic gemmation do not ordinarily throw off sexual zooids, and that species in which the latter phenomenon occurs do not usually furnish instances of proembryonic forms.

§ 8. Continued Pullulation in the same stage.

The regularity in the alternation of free zooids with true embryos is frequently obscured in nature by the intervention. of a process of pullulation, or budding off of like forms, in continued succession, at some particular stage in the life-history of the species, so that sexual zooids recur only at intervals, separated by periods during which a series of neuter forms occur of the same general character, if not all absolutely alike. In some cases, the number of interpolated links appears to be fixed; but in general it is variable, and frequently the recurrence of the sexual form which closes the series seems to depend on circumstances, the true ova being commonly formed on the approach of winter, or other conditions adverse to the continuation of active vitality.

*Allman's British Fresh-water Polyzoa (Ray Soc.), p. 41.

A course of pullulation may be thus interpolated at any stage. It is met with in the germinal stage in the case of the Trematoda, and in the gamomorphic or sexual stage in a few medusoids; but more commonly it occurs in the orthomorphic stage, being interposed between the first appearance of the typical characters, and the development of the structures which originate the sexual elements. In fact, the orthomorphic gemmation, just noticed as one form of alternation, almost always runs on into a continued course of pullulation, the result being either a swarm of free zooids, as in the case of the Aphides, or else a composite structure, like the leafy stem of a plant, or the polypidom of a zoophyte. The latter alternative is the more common; for the tendency of the gemmæ, in most cases of continued pullulation, is to remain during their whole term of life in connection with the parent stock, either directly, or through the medium of their predecessors in the series of offshoots.*

9. Protomorphic Alternation in relation to Embryogeny.

Though the well-marked cases of alternation, due to the evolution of protomorphic zooids, are confined to a few of the lower orders, a certain nisus or tendency in this direction-a fresh start, as it were, in the course of germinal developmentmay be traced with more or less distinctness in all cases of embryogeny, as in all instances there is formed first a cellular germ-mass, from one point of which there is subsequently developed a new axis of embryonic growth.

The embryo, in short, may be said to be budded off from the primordial germ-mass, much as the larval distoma is from the gregariniform product of the Trematode ovum. There are, however, two points of diversity. In normal development, the germ-mass gives rise only to a single embryo, and no separation takes place between them. The later growth appears simply as a more advanced state of the former, which wastes

* In the tabular views of the Genetic Cycle, given at the end of this article, such continued pullulation-as being only an occasional phenomenon-is printed in a smaller type.

away, pari passu, with the growth of the embryo, becoming a mere appendage of the latter, or disappearing altogether. In alternation or metagenesis, again, the immediate product of the ovum gives rise to numerous gemmæ, every one of which may acquire the characters of a typical individual of the species; and we find that these gemmæ generally become completely separated from their germ-parent, and assume the form of independent organisms. But although the detachment of the later growth, and its multiplication, give an apparent distinctness to the cases in which they occur, there are yet phenomena of an intermediate kind, which indicate a certain community of nature between them. Such are the following:

1. The duplication, in whole or part, of the embryonic axis, as an occasional abnormality, even in the higher species, resulting in the formation of a double monster.

In the eggs of the pike, according to M. Lereboullet," the formation of these monsters may be determined at pleasure, by placing the eggs in unfavourable conditions for development." In this case the blastodermic ridge forms on its surface two tubercles instead of one, and from each of these an embryonic fillet is produced, the further development of which gives rise to double embryos of various kinds.*

2. The regular formation of a double embryo from the ovum, in the case of the Polyzoa.

Here the immediate product is a ciliated germ-mass, like an infusorial animalcule, from a protrusion of which, according to Professor Allman, a pair of polypes are budded off in succession, the process presenting, as he observes, some remarkable analogies, tending to bring the whole process of gemmation and generation within the domain of the so-called "law of alternation of generations," though neither the two first-formed gemmæ, nor those which afterwards pullulate from them, ever become detached, while the original germ-mass becomes as completely reduced to the condition of a mere appendage of the

* An. Nat. Hist., 2d Series, xvi., 49.

British Fresh-water Polyzoa (Ray Soc.), pp. 41, 33, 34.

structures derived from it, as in the case of the ovum of any vertebrated animal.

3. The variable character of the gemmation of Tæniaheads in the cystic Entozoa-solitary in the Cysticercus, but multiple in the case of the Conurus and Echinococcus.*

4. The co-existence among the Echinodermata of cases resembling ordinary embryogeny, or the metamorphosis of insects, as in Echinaster or Holothuria, with others, constituting the majority of the class, in which the embryo, though still solitary, stands out as a distinct structure from the so-called larva, and has in so far the character of a derivative zooid.

The case, therefore, seems to stand thus. Embryonic gemmation may be said to occur in all cases, though in the higher animals only in a latent form; while in the lower species, it is, so exaggerated as to acquire a wholly new character. So long as the exaggeration is merely in its distinctness, or in the more complete detachment of the gemma, the affinity of the process to the normal course of embryogeny is sufficiently apparent (as in the Echinodermata). Even when a new element of discrepancy is introduced by a multiple gemmation, we can still find a parallel in the embryogeny of the higher animals, though now only as an occasional abnormality. But when the breach is yet further widened by one or more repetitions of the process of gemmation, we have results so totally unlike the ordinary course of reproduction in the majority of animals, that it is with some difficulty we can realize any community between them.

§ 10. Gamomorphic Alternation in relation to Sexual

Maturation.

As the appearance of a new centre of organisation in the cellular germ-mass may stand in the higher species as a representation of protomorphic alternation, so to the contrasted form-marked by the formation of sexual or gamomorphic

* An argument of all the greater cogency if Conurus be, as Siebold contends, a mere variety of Cysticercus. Even in admitted forms of Cysticercus, how ever, such multiple gemmation of Tænia-heads has been observed.

NEW SERIES.-VOL. XI. NO. I.—JAN. 1860.

B

zooids-may we trace a certain correspondence in the maturation of the reproductive organs. Such a correspondence is

suggested in particular by the following considerations:

1. The periodicity and lateness of development of the organs of reproduction in most species, and their greater or less independence of the rest of the system in some cases. This is so much the case in the Polyzoa, that, in the opinion of competent judges, they hold the position rather of derivative gemmæ, than of mere organs of the particular polypes, in connection with which they are developed. Thus, Professor Allman remarks, "If the formation of the ovary be attended to, it will be seen that this body is developed at a later period from the walls of the original sac-like embryo, which have undergone slight changes, and have become the endocyst of the more mature polyzoon, and it will be at once perceived that this development of the ovary takes place in a way which may obviously be compared with the formation of a bud; that at least in Alcyonellait occupies exactly the position in certain cells that the buds destined to become polypides [polypes] do in others, and that, at an early stage of polypide and ovary, it is scarcely possible to distinguish one from the other; so that the idea is immediately suggested, that the body here called ovary is itself a distinct zooid, in which the whole organisation becomes so completely subordinate to the reproductive function as to be entirely masked and apparently replaced by the generative organs." On similar grounds he argues, that the spermatic organ" may perhaps be more correctly considered, like the ovary, as a distinct sexual bud, having the generative system so enormously predominant as to overrule and replace all the rest of the organisation; this bud, like the ovary-bud, being also unisexual, but with a male function."

2. The transition in certain families, as the Polypifera, and even among closely allied species, from cases in which the reproductive organs are integral parts of the system and of very simple structure, to others in which they occur in detached zooids, having the character of distinct and well-organised animals.

3. The accidental nature of the characters which principally