INFUSORIFORM EMBRYOS. 135 



of the urn lie at first side by side, but in the course of develop- 

 ment 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 infusori- 

 form 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-forma- 

 tion 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 pro- 

 cess in Dicyema probably indicates that the hypoblast was primi- 

 tively multicellular. It is not improbable that the enigmatical 

 infusoriform embryo may develop into a sexual form, the pro- 

 geny of which are destined to complete the cycle of develop- 

 ment by becoming again parasitic in the renal organ of a 

 Cephalopod. 



BIBLIOGRAPHY. 



(117) E. van Beneden. " Recherches sur les Dicyemides." Bull. d. FAca- 

 dtmie roy. de Belgique, i" ser. T. XLI. No. 6 and T. XLII. No. 7, 1876. Vide this 

 paper for a full account of the literature. 



(118) A. Kolliker. Ueber Dicyema paradoxum den Schmarotzer der Venenan- 

 hdnge der Cephalopoden. 



(119) Aug. Krohn. "Ueb. d. Vorkommen von Entozoen, etc." Froriep 

 Notizen, vii. 1839. 



