42 C. 0. Whitman 



light , thus preparing the way for discriminating- between them even 

 when intermingled . I have seen cases which, at the time of recording, 

 seemed to me to favor the idea of a transition from the Nematogen to 

 the Rhombogen; but I am uow of the opinion that the transition is al- 

 ways in the opposite direction. 



Transition al Phase s. — The following cases are given as 

 illustrations of what may be seen during the transiti onal period. 



1 . Dicyemennea Eledones from Eledone moschata^ Mar. 3 1 . Fig. 15, 

 PI. 1 , represents a portion of a large individuai, in which are seen the 

 central nucleus (we), two paraunclei (w'), two Infusorigens , and scat- 

 tered cells [vg). No embryos nor embryonic stages. The two Infusori- 

 gens show all the parts of active Infusorigens, but their peripheral cells 

 no longer exhibit the compact arrangement characteristic of earlier 

 stages. 



Largest peripheral cells .016 mm in diam., nuclei .01 mm — .012mm 



Largest scattered cells .012mm- - nuclei .004mm — .005mm 



The difference in size between the nuclei of the two classes of cells 



is worthy of notice. The smallest cells of both kinds agree in every 



particular ; the nuclei of the larger scattered cells differ but little from 



those of the smaller cells. 



The paranuclei differ from the central nucleus only in size and 

 form. Both present the thick membrane, the reticulum, and large vac- 

 uolated nucleoli. The germogen shown a residual nucleus. and, in one 

 case, a rosette of six or more nuclei (?). 



Not a single individuai was found with infusoriform embryos , nor 

 any approaching the pure Rhombogenic condition more nearly than the 

 one bere given. Many were seen in which the entire axial cell was 

 filled with loose or scattered cells like those seen in the fig. [vg]^ and 

 others differing from these only in having from one to many vermiform 

 embryos. 



I regard this as a Rhombogen already far advanced towards the 

 Nematogenic state. The infusoriform embryos have ali escaped, and 

 the Infusorigens are in progress of breaking up info scattered cells. 



2. D. gracile, from Sejna officinalis^ Mar. 30. In two iudividuals 

 vermiform embryos (twoineachcase) were found, and with them several 

 Infusorigens accompanied by early stages of infusoriform embryos. 

 Most individuals obtained from this Sepia were Rhombogens. 



These two instances are the only ones I have ever met with; in 

 which I could say with certainty that the two kinds of develop- 



