Each phase in the evolution of this primitive vertebra has been attended by degeneration. In man the vertebral column consists of :(a) Seven cervical vertebrae.

(b) Three dorsal vertebrae.

(c) Five lumbar vertebrae.
(d) Three sacral vertebrae.

(e) Six coccygeal vertebrae.

Each one of these vertebrae exhibits important modifications, and shows signs of degeneration. The proatlas is represented by its intercentrum only. The atlas consists of a centrum and neurapophyses, but there is no zygapophyses; it possesses one pair of ribs and hæmapophyses in a reduced condition. The axis consists of the same elements but carries postzygapophyses.

The four following cervical vertebrae consist of the same elements, but carry both zygapophyses and postzygapophyses. In man all the cervical vertebrae, with the exception of the first (the proatlas) have lost the intercentrum. The next vertebrae which is generally regarded as the seventh cervical vertebra, consists of the same elements, but it ought to be regarded as the first dorsal vertebra. The vertebral artery and the sympathetic nerve trunks accompanying it, do not pass through the vertebral canals; in some cases these do not exist. The so-called seventh cervical vertebra has occasionally one fully developed rib which articulates with the sternum like the ribs of true dorsal vertebrae.

With the

There are thirteen dorsal vertebrae. exception of that which has just been described, they consist of:-a centrum, neurapophyses, and a pair of fully developed ribs; the intercentra has completely disappeared, and the hæmapophyses, which form the head and neck of the rib, are in a reduced condition. In the five lumbar vertebrae which follow, the rib disappears, or to speak more accurately, the transverse processes are all that remain of what were the ribs, and have ossified with the vertebra.

The sacrum is a region profoundly modified to support the pelvic basin. It is formed by the fusion of five vertebrae, each consisting of a centrum, neurapophyses, and short bicipital ribs. The first three vertebrae are the true sacrals, as these alone support the basin; the two following are really caudal vertebrae in process of fusion with the sacrum. In monkeys other than anthropoids there are really only three sacral vertebrae and these are at once succeeded by the tail. In man, the tail 1 consists of six vertebrae of which the two firstwhich consist of a centrum and neurapophyses and of bicipital ribs-have fused with the sacrum, while the four remaining lower vertebrae, which consist of only the centra (the first still exhibits rudimentary neurapophyses) have fused, and form the coccyx.

1

1 See Albrecht, La queue chez l'homme (Bull. soc. Anthrop. Brux., vol. iii., 1884-1885.

2. The Frog.-The vertebral column in the frog consists of nine vertebrae and the coccyx (urostyle).

The first vertebra (the proatlas), which is fully functional, retains a centrum and two well-developed neurapophyses, but the transverse processes, the intercentrum, the hamapophyses and the ribs have all disappeared.

The eight following vertebrae each have a centrum, neurapophyses and transverse processes which at least partially represent the ribs.

The coccyx, whether it be formed by the lengthening out of the last caudal vertebra or by the fusion of several, is undoubtedly part of the vertebral column which has been transformed. The coccyx of the frog is equal in length to the whole of the remaining part of the vertebral column, and is fully functional; it serves as a support to the pelvic region and fulfils the part of a sacrum from the physiological point of view. The coccyx, excepting at its commencement, consists of only one centrum, or of several fused centra, all other elements of the vertebrae have disappeared. Here then is an animal in which the modifications of the vertebral column have been attended by the following retrogressive phenomena : in the upper part of the vertebral column from three to five parts of the vertebrae have disappeared; in the lower half, all the parts, excepting one, are gone.

SECTION II.

Rudimentary Organs in Plants.

We have just glanced through a series of rudimentary organs in animals, and many more examples might easily have been furnished, but, when dealing with rudimentary organs in the vegetable world, much greater difficulty is met with. In plants, the elimination of non-functional organs is usually complete, and the vestiges left are insignificant and hard to recognize. We can find, however, amongst the various groups of the vegetable world, and especially among the Phanerogams, some instances of reduced orgáns.

§ 3. Rudimentary organs in various groups of plants.

1. Algae. On the surface of sea-wrack (Fucus) may be found, distributed in large numbers, little crypts (conceptacles) with hairs growing out of them. On certain parts of the plant, these crypts represent the organs of reproduction, producing eggs and spermatozoa; in other parts they fulfil no known function and may be regarded as conceptacles arrested in the course of development. The fact that in other Fucacia (Splachnidium), fertile conceptacles are distributed over the entire surface of the plant adds support to this theory.

2. Mushrooms. Among the Peronospora and the Saprolegneæ there originally existed, besides the various sexual means of propagation, a typical reproductive process, including eggs, and antheridia, consisting of male branches separated by a cellwall from the rest of the organism.

In Pythium, for instance, in which this primitive stage may be observed, an actual fecundation takes place, the protoplasmic contents of the antheridia being injected into the ova.

In other species, the organs of reproduction have undergone a more or less complete degeneration. In Phytophthora, a small part only of the male protoplasm passes into the ova. In some species of Saprolegnia and Achlya, the male branch continues to attach itself to the ova, but the membrane between them remains intact, and consequently protoplasmic communication is not established.

In other species, the antheridia are very short, and do not even touch the female cells.

In Leptomitus, which exhibits an advanced stage of atrophy, the female organs are not discernible, and reproduction is carried on completely asexually.1

3. Bryophyta. In the germination of a certain number of Hepaticæ, belonging to such widely

1 For further details see A. de Bary, Vergleichende Morphologie und Biologie der Pilze, Leipzig, Engelman, 1884. W. Zopf, Die Pilze. In Schenk's Handbuch der Botanik, 4. Bd., Breslau, Trewendt, 1890.