l'hypothèse d'après laquelle les matériaux destinés à fournir à la moitié droite du corps siègeraient dans l'un des hémisphères latéraux de l'œuf, tandis que l'hémisphère ovulaire gauche engendrerait tous les organes de la moitié gauche du corps."

If our ideas of the promorphological relations of the ovum are well founded, it will be seen that we have a very satisfactory foundation for the opinion, first suggested by Balfour (Comp. Emb., II., p. 312), that the neural surface is identical throughout the metazoa. It is hardly necessary to add, that this view is in perfect accord with the theory of concrescence before mentioned. Indeed, it is difficult to see how one can hold to the former, and deny the latter.

EXPLANATION OF FIGURES.

Figs. 1-5, Ctenolabrus. Fig. 6, Ps. oblongus.

All magnified 280 diameters.

Fig. 1. Blastodisc seen from the inner surface. The Arabic numerals give the order of the cleavage-planes; the shading indicates that portion of the floor of the marginal cells which rests on the yolk. a, b, c, d, central cells; cl, boundary of the cleavage-cavity; xy, plane of the vertical section seen in Fig. 2.

Fig. 2. Transverse section of the 16-cell stage, in the plane indicated by the dotted line (xy) in Fig. 1. p, periblast; bc, cleavagecavity.

Fig. 3. Section of the blastodisc one hour after the 16-cell stage. The marginal cells are shaded.

Fig. 4. Two hours after the 16-cell stage. ep, epidermal layer. Fig. 5. Three hours after the 16-cell stage. The marginal cells assuming the form of periblastic cells.

Fig. 6. Transverse section at the time when the entodermic ring appears. en, basis of the future entoderm.

V.

CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF THE MUSEUM OF COMPARATIVE ZOOLOGY

AT HARVARD COLLEGE.

No. IV. THE EXTERNAL MORPHOLOGY OF THE LEECH.

By C. O. WHITMAN.

Communicated June 11th, 1884, by Alexander Agassiz.

THERE is no invertebrate animal about which more has been written than about the Medicinal Leech; but, as Dalyell long ago remarked, "it does not appear that the history of the leech has advanced in proportion to the number of the literati who have rendered it the subject of discussion." As a considerable share of the work done in this direction is purely systematic, it is somewhat surprising that not a single description of any Hirudo has been given with sufficient accuracy and completeness for a close comparison of even its more important external characters with those of other species. More than this, it would be impossible, from the many monographs, memoirs, and stray papers devoted to this subject, to patch up a description that would fully meet the requirements for a critical comparison of any two species. By far the greater number of species-diagnoses that have been showered upon us from time to time have been so superficially and slovenly done, that it would probably puzzle the perpetrators to identify the species they profess to have described. Some of the more important diagnostic characters have been either entirely ignored, or given with such vagueness that they are of little service in identification, and absolutely worthless for comparative purposes. No uniform mode of counting the rings has been adopted; and, judg ing from the descriptions themselves, systematic writers have, for the most part, failed to place much value on the rings in the determination of species and genera. No one appears to have suspected the existence of segmental sense-organs in the leech; much less the serial homology of the eyes with such organs. It is the object of this paper to make clear both these facts; and, at the same time, to show that the rings and somites form the only proper basis of classification. The latter fact will be brought out by a comparison of a few wellmarked genera.