The analogies which have been under our consideration in the preceding parag,raphs, may be placed in a clearer point of view, by exhibiting them in a tabular form. Referring the reader to the description contained in OS. of the most simple form of unicellular conjugating Algm (PaltnogIcea macrococca), we shall contrast the circle of development, as it presents itself in PalmogIcea and Proto coccus on the one hand, with that of Vorti cella on the other, as follows Here it may be observed that in the stage of cessation of growth, which, in the Pro tozoon, as well as in the Protophyton, follows the act of conjugation, we have a condition which corresponds to that of the ovum of the higher animals. The ovum after passing through a period of repose, resembling that which presents itself in Podophrya, exhibits a series of transformations, which correspond to the later steps of the developments under our consideration. This correspondence is, as might be expected, more distinctly seen in the lower than in the higher animals. Thus for example, in the development of a Trema tode Worm (Distomum pacificum), the mass of the yolk is transformed into a locomo tive rudiinent resembling an infusory animal. Within this originates an asexual, but fertile nurse, the homologue of the Vorticella, in the interior of which is formed a second and numerous generation of animals endowed with locomotion (Cercarim). In these, after
a time, the locomotive power is lost, and each finally becomes a sexual Distomum.* Although the foregoing homologies are founded on observations the details of which are as yet imperfectly worked out (on which account it may seem somewhat premature to draw attention to them), they are not open to the objections which may be urged to homo logies supposed to exist between the highest members of the two series. There, the con necting links are wanting; here, we pass through closely related intermediate forms, froin the Alga to the Protozoon, and from the Protozoon to the Trematode Worm. Hence, while we are not justified in applying the term ovum to the generative product of the phanerogamous plant, the present state of our knowledge allows us with propriety to compare with the ovum the result of con jugation as it occurs among the Algx.
The differences in chemical composition which exist between the Algm and the Pro tozoa will not serve as a ground of distinc tion. Euglena is invested during its period of repose with a cellulose membrane and contains granules of chlorophylle. In Poly toma uvella we find, on the one hand, the contractile vesicles of the infusory animal, on the other, starch in the granular form, so characteristic of the plant.t 7 71 el 7