NUCLEAR CONSTANCY IN HYDATINA. 91 



those in the anterior region of the body being identical in form 

 with those just described for the Neoechinorhynchidse while those 

 in the posterior region are coarsely dendritic in nature, composed 

 of an elongated central mass from which a considerable number 

 of projections are given off along the channels of the lacunar 

 system. Nuclei of this second type are distinctly intermediate 

 between the form characteristic of the Neoechinorhynchidse and 

 those described for the species Echinorhynclms thccatns. In this 

 last named species the subcuticular nuclei are finely, dendritic, ex- 

 tending as fine branches over a considerable area. In some 

 species of the genus Echinorhynchus and of the genus Pompho- 

 rh\nchus this dendritic condition has gone to such an extreme 

 that the small nuclear masses are joined by only fine threads and 

 in many places these communicating threads have not been de- 

 tected. In this last named condition the nuclear material has the 

 appearance of being distributed in small isolated nuclear masses 

 throughout certain regions of the subcuticula. Thus in this 

 group of parasitic worms the subcuticular nuclei present a finely 

 graded series of changes from the simple rounded or ovoid nuclei 

 retained directly from the embryonic condition to a highly frag- 

 mented condition in which there is little or no regularity in the 

 numbers and arrangement of the particles resulting from the 

 fragmentation. Consequently in Neoechinorhynchidse there is a 

 constancy in the number of the subcuticular nuclei throughout the 

 life of the individual while in the most of the other Acanthoce- 

 phala late embryonic or post-embryonic fragmentation of the 

 nuclei disrupts the constancy in numbers of histological elements. 

 That a similar amitotic division might be the explanation of 

 increased numbers of nuclei in the aberrant vitellaria of Hydatina 

 senta seems highly probable. Moreover, the abnormal individ- 

 uals observed in this study seem to indicate that definitely local- 

 ized stimuli operate in inducing further divisions of the nuclei, 

 for not all of the nuclei of the vitellarium are affected. In all 

 three of the aberrant specimens observed the increase in the num- 

 ber of the nuclei has apparently been in the region of the vitel- 

 larium immediately in contact with the ovary. The exact nature 

 of the stimulating agent has not been determined but it seems 



