Fig. 62. Infusoriform embryo of Dicyema.
A. B. C. Three of the later stages in the development.
D. E. F. Three different views of the full-grown larva. D. from the front, E. from the side, and F. from above.
G. side view of urn.
u. wall of urn; l. lid of urn; r. refractive bodies; gr. granular bodies filling the interior of the urn.
The primitive cell is called by Van Beneden a Germogen. In its protoplasm a number of germs first appear endogenously, but the nucleus of the germogen does not assist in their formation. They eventually become detached from the parent cell, around which they are concentrically arranged. A second and then a third generation of germs are formed in the same way, till the whole of the protoplasm of the primitive cell is absorbed in the formation of these germs, and nothing of it remains but the nucleus. The germs so formed are arranged in about three concentric layers, of which the innermost is the youngest. One to five masses of germs may be present in a single Rhombogen. The germs undergo a division, in the course of which their nuclei exhibit very beautifully a spindle modification. In the course of the segmentation the embryo gradually assumes its permanent form, and four of the cells composing it can be distinguished from the remainder by their greater size ([fig. 62] A, u). The two largest of these give rise to the wall of the urn, and also give origin to four smaller cells ([fig. 62] B, gr) which eventually become polynuclear and constitute the four granular cells in the urn. The two other cells become the lid of the urn. The parts of the urn lie at first side by side, but in the course of development the cells which form the wall of the urn travel inwards, and the four granular cells are carried into their concavity. At the same time the cells which form the lid of the urn alter their position so as to overlie the wall of the urn. The two cells immediately above the urn give rise to the refractive bodies ([fig. 62] A, B, C, r) and the remainder of the cells of the embryo become the tail ([fig. 62] C). The embryo becomes ciliated, and attains its nearly full development before leaving the parental tissues. It usually passes out at the cephalic extremity.
As has already been stated, it is probable that the infusoriform embryos leave the renal organs of their host and lead a free existence. What becomes of them afterwards is not however known, though there can be little doubt that they serve to carry the species to new hosts.
Till the further development of the infusoriform embryo is known it is not possible to arrive at a definite conclusion as to the affinities of this strange parasite. Van Beneden is anxious to form it, on account of its simple organization, into a group between the Protozoa and the Metazoa. It appears however very possible that the simplicity of its organization is the result of a parasitic existence; a view which receives confirmation from the common occurrence of the process of endogenous cell formation in the axial hypoblast cell. It has been clearly shewn by Strasburger that endogenous cell formation is secondarily derived from cell division; so that the occurrence of this process in Dicyema probably indicates that the hypoblast was primitively multicellular. It is not improbable that the enigmatical infusoriform embryo may develop into a sexual form, the progeny of which are destined to complete the cycle of development by becoming again parasitic in the renal organ of a Cephalopod.
Bibliography.
(117) E. van Beneden. “Recherches sur les Dicyemides.” Bull. d. l’Académie roy. de Belgique, 2e sér. T. XLI. No. 6 and T. XLII. No. 7, 1876. Vide this paper for a full account of the literature.
(118) A. Kölliker. Ueber Dicyema paradoxum den Schmarotzer der Venenanhänge der Cephalopoden.
(119) Aug. Krohn. “Ueb. d. Vorkommen von Entozoen, etc.” Froriep Notizen, VII. 1839.