I5 2 THE RHOMBOZOA 



embryo ; (3) of an " urn," which is partially enclosed in the preceding, 

 but comes to the surface antero-ventrally. This urn, in its turn, is made 

 up of (a) a " lid " or cover of four non-ciliated cells, which completes 

 the outer wall of the embryo ; (ft) an " urn -wall " formed of two curved 

 cells, laterally placed, underlying the ciliated cells ; and of (y) the 

 " urn-contents " namely, four granular cells, arranged crosswise ; each 

 has at first a single nucleus, but later many nuclei. According to 

 van Beneden, these nuclei belong to minute cells provided with cilia, and 

 he suggests that they are spermatozoa. This elaborate organism, or 

 individual, arises in a very peculiar manner. Each germ cell divides, 

 first of all, unequally, the smaller cell taking no further share in 

 the process, but its nucleus enlarges, and lies freely in the axial cell ; 

 it is the " paranucleus " (Whitman), and is very suggestive of a polar 

 body of metazoa in general. The remaining cell proceeds to divide, 

 very much as in the case of the vermiform embryo one large cell 

 becomes surrounded by a layer of smaller ones. This gastrula-like phase 

 is termed an " infusorigen " by Whitman, and the central cell is the 



Fio. VI. 



Infusorifonn embryo (or the male) of Dicyemn 

 (after van Beneden). A, from the right side; 

 B, from the ventral surface ; r, the refringent 

 body ; u, the capsule or wall of the urn ; c, the 

 contents of the urn (granular bodies); I, the 

 lid; b, ciliated cells of the body wall (ecto- 

 derm). 



" germogen." But this germogen proceeds to divide, endogenously, much 

 as the axial cell of a vermiform embryo does, so that its nucleus becomes 

 surrounded by a second generation of nuclei, each of which becomes 

 invested by protoplasm to form a cell. The "ectoderm" cells of the 

 infusorigen separate, and each develops into an infusoriform embryo, 

 which ultimately escapes from the rhombogen. The remaining cells 

 also separate, but develop into vermiform embryos, the central nucleus 

 (germogen) remaining behind free in the axial cell of the parent as a 

 M residual nucleus " (Whitman). 



The rhombogen, after the birth of the infusoriform embryos, becomes 

 a "secondary nematogen," containing vermiform embryos. Thus there 

 are monogenetic individuals (primary nematogens) and diphygenetic in- 

 dividuals which produce, firstly, infusoriform, and later, vermiform 

 embryos. The infusorigen is an embryo which gives origin to other 

 embryos, much as has been shown to occur in Gyrodactylus. The in- 

 fusoriform embryos escape from the parent, and, unlike the vermiform 

 embryos, can live in sea- water (Erdl, van Beneden) ; but no changes of 

 any kind have been observed in them, and their later fate is doubtful. 

 But van Beneden, influenced by MetschnikofFs discovery of sexual dimor- 

 phism in the Orthonectida, and by Julin's observation of the two forms 

 of females, suggested that the " infusoriform embryos " of Dicyemids are 

 males. 1 Moreover, Whitman has occasionally observed in large specimens 



1 This appears to be confirmed by Wheeler (Zool Anzeig. xrii. p. 169, 1899), 

 who states that male dicyemids, i.e. infusoriform embryos are produced from 



