A Contribut. to the Embryol., Life-history, and Classificai, of the Dicyemids. 41 



it is the Substitution of the Nematogenic for the Rhombogenic mode of 

 reproduction. The individuals in which such a transition has abeady 

 taken place may be called seconda ryNemato gens, and these may 

 be easily distinguished from primary Nematogens by the occur- 

 rence of more than one large free nucleus in the axial cell. 



Transitional Period. — The passage from one state to the 

 other, in cases where the number of Infusorigens is more than one, may 

 not begin simultaneously in these several centers ; so that one portion 

 of the individuai may be occupied by one or more Infusorigens and 

 various stages of infusoriform embryos , while another portion is filled 

 with germ-cells destined to produce eventually vermiform embryos. 

 Thus the two modes of cell-genesis may co-exist in the same individ- 

 uai; but this co-existence never, so far as I bave seeu , continues so 

 long that the two kinds of embryos occur together. The infusoriform 

 embryos desert the parent before the development of vermiform embryos 

 begins. 



The disappearance of the infusoriform embryos does not however 

 bring the transitional period to a dose. This period extends from the 

 moment when, in any one of the Infusorigens, the production of infu- 

 sorific germs ceases to the time vrhen the proper vermific germ-cells 

 appear, and the latter arise only after a considerable time has been 

 consumed in the multiplication of cells by simple division. Essentially, 

 it is a germ-producing period throughout , complicated only at the be- 

 ginning by the co-existence of two modes of cell-genesis. 



Simple as the history of this period is, it has absorbed more time 

 than any other portion of my work. Evidences tending to establish the 

 fact of a transition accumulated for a considerable time , before it be- 

 came clear in what direction it takes place. If the dissemination of the 

 species is provided for b}' the infusoriform embryos, it seemed naturai, 

 assuming that a change of generation takes place , to expect that these 

 embryos would arise after the vermiform. 



It seems from a citation before given, that this order of events was 

 among the possibilities that occurred to Van Beneden, although he 

 went no further than to simply state the question. 



The co-existence of the two modes of generation in the same in- 

 dividuai made it somewhat difficult to obtain a clear notion of the dis- 

 tinctive features of the Rhombogenic and Nematogenic conditions. The 

 study of the development of the Rhombogen and the Nematogen, and a 

 comparison of the two states in adult stages , gradually sharpened the 

 contrast between them, and placed their essential characters in a clearer 



