DICYEMID.E. Ill 



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 develope 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. "Eecherches sur les Dicyemides." Sail. d. V Academic 

 roy. de Belgique, 2 e ser. T. XLI. No. 6 and T. XLII. No. 7, 1876. Vide this paper for a full 

 account of the literature. 



(118) A. KSlliker. Ueber Dicyema paradoxum den Schmarotzer der Venenanhdnge 

 der Cephalopoden. 



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

 vii. 1839. 



