A Conh-ibut. to the Eiubryol., Life-history, and Classificai, of the Dicyeraids. 53 



celi , the parallel between the Infusorìgen and the vermiform embryo 

 would he too complete to leave any roora for doubt. No explanation 

 can, however, be accepted, which fails to account for the fact that the 

 history of the Infusorigen comprìses two distinct periods ; namely, a 

 developmeutal period, in which a gastrula-like body is produced 

 by a process comparable to ci eavage, and a r e p r d u e t i v e period, 

 in which the germ-cells arise endogenously in a single centrai celi. 



There is another point which mnst be uoticed in this connection. 

 How are we to explain the cohesiou of the peripheral cells on any sup- 

 position that ignores or denies the individuality of the lufusorigen? If 

 these cells bave no organic relation with one another, why do they 

 adhere to the germogen and to each other by piane faces? If there is no 

 bond of union between theni more than exists among independent germ- 

 cells, why does there invariably arise a gronp , like that seen in figs. 

 109 and 110? Nothing of ali this is seen among the germ-cells of the 

 Nematogen. So long as germ-cells multiply by simple division, they 

 show no disposition to arrange themselves into regulär groups ; the 

 products of division at once separate, and if they come into contact with 

 others, their independence is stili attested by their spherical form. The 

 whole axial cell may be crowded with such cells , and nowhere any 

 semblance of an organic assemblage. The cells are like so many 

 marbles confìned within a certain area, in free rolling contact. As soon 

 as developmeutal division begins in any one of these cells, a group is 

 formed , which eventually becomes a vermiform embryo. We bave seen 

 also that the Rhombogenic stage is preceded by one in which the germ- 

 cells maintain the freedom characteristic ofvermific germ-cells. As soon 

 as one of these cells has eliminated the nucleus-like corpuscle which 

 we bave called paranucleus , a kind of developmental division begins 

 and a group is formed to which we bave applied the name Infusorigen. 



If the Infusorigen arose by endogenous cell-formation, the cohesion 

 of its peripheral cells for a short time would require no further explana- 

 tion than a mere description of their mode of origin. Their adhesion to 

 each other and to the germogen would be simply the resnlt of mechan- 

 ical pressure, which further growth would overcome, and thus lead to 

 complete Separation. Such, presumably, was the view taken by Van 

 Beneden, since no comments were added to the follo wing statements : 



»Ils [the peripheral-cellsj ne se détachent que quand ils ont atteint 

 leur complet dévelopement. Tant qu'ils adhèreut au germigène, la sur- 

 face de contact est piane : les gernies ont tous la forme d'une sphère 

 tronquée et le germigène est limite par des faces planes se coupant 



