ORTHONECTIDiE AND DICYEJIID.E 



213 



a process which Whitman compares to the formation of 

 the polar globules in the eggs of the Metazoa. As the result 

 of a process of division a considerable portion of the nucleus 

 is said to be cast out of them, which, as the " paranucleus," 

 can be recognized for a long time in the axial cell (Fig. 101 

 B). Then ensue a cleavage of the cells and, as its result, 

 the formation of cell-masses which have quite the appear- 

 ance of an epibolic gastrula with a central cell. Such stages 

 had already been observed by van Beneden (Pig. 101 A). 

 They are entirely like those which occur in the development 

 of the vermiform embryos. Whitman compares them directly 

 to these, and looks upon them as 



special individuals, which appear 



early in the course of reproduction. 



For in their central cells there are 



soon formed new cells (Fig. 101 A and 



B), which subsequently give rise to 



the infusoriform embryos. On this 



account Whitman calls this gastrula 



stage an Infusorigen. Compared to 



the nematogenic developmental series, 



the gastrula stages would correspond 



to the vermiform embryos, which, as 



we saw, also produce embryos at a 



very early period. 



From the central cell (cellule ger- 



migene of van Beneden) of the gas- 

 trula stage, which increases in size, 



arise several generations of germ 



cells, which surround it in the form 



of a rosette. 1 The larger of these 



cells become infusoriform embryos ; 



the smaller ones are said subsequently 



to divide repeatedly, and vermiform 



embryos are said to arise from them 



when, after the formation of the in- 



1 The central cell itself is to be looked 

 upon as the homologue of the axial cell of 

 the vermiform embryos. 



Fig. 101. — ^, "Infuso- 

 rigen embryo " (after van 

 Benbdbn) ; B, the same 

 lying in the axial cell (Ax) 

 of the rhombogen indi- 

 vidual (after Whitman). 

 A, of Dicyema typus; B, of 

 Dicyemennea eledones. C, 

 the central cell of the " In- 

 fusorigen embryo," which 

 has already produced new- 

 germ cells ; K, nucleus of 

 the central cell ; Ke, nuclei 

 of the outer layer of the 

 rhombogen individual ; Pn, 

 paranucleus. On the right 

 side of Fig. 101 B the re- 

 ferences K and Ke are 

 transposed. 



