ORTHONECTID^ AND DICYEMID^ 



213 



a pi'ocess 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. lUl 

 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 (Fig. 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 

 vei'y early period. 



From the central cell (cpllide 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.^ The larger of these 

 cells become infusoriform embi'yos ; 

 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- 



' The central cell itself is to be looked 

 upon as the homologue of the axial cell of 

 the vermiform embryos. 



Fig. 101. — vl, "Infuso- 

 rigen embryo " (after van 

 Benedbn) ; B, tbe same 

 lying in the axial cell (^a.) 

 of the rhombogen indi- 

 vidual (after Whitman). 

 A, ot Dicyema typus; B, of 

 Bicyeniennea eledones. C, 

 the central cell of the " In- 

 fusorigen embryo," whicli 

 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. 



