FORMATION OF PROGLOTTIDS IN CROSSOBOTHRIUM. 213 



17, VI., and 22, VII.), and this segmentation progresses from be- 

 hind forward as did the formation of " posterior segments " pre- 

 viously described. When from six to twelve such segments 

 have been established there begins a segmentation of the front . 

 end of the neck. This proceeds from in front backwards as did 

 the formation of " anterior " proglottids at an earlier stage. 

 Specimens showing the above changes are shown in Figs. 17, PI. 

 VI., 22, PI. VII., 20, PI. VI., and 24, PL VII. The number of 

 proglottids of the old chain which are present when the neck be- 

 gins to appear and later the number present when the segmenta- 

 tion of the neck begins, appears subject to a considerable varia- 

 tion. For example, Fig. 15, PI. VI., and Fig. 21, PI. VII., are 

 specimens which when the state of contraction in each case is 

 considered appear to have the neck developed to about the same 

 extent, but the number of remaining proglottids is ten in one case 

 and thirty-four in the other. Nothing can be determined on this 

 point from Figs. 17 and 18, PI. VI., or 22 and 24 of Plate VII., 

 for although Figs. 18, 22 and 24 had the number of proglottids 

 indicated when they were taken from the shark they may have 

 been mutilated when the valve was cut open, while in the case of 

 Fig. 17 I have no record of the condition when collected and 

 hence suspect the possibility that some segments may have been 

 detached by accident. Again, in Fig. 20, PI. VI., is shown a 

 specimen in which the neck has both anterior and posterior seg- 

 ments though forty-nine of the old proglottids remain. This is 

 the maximum number of the old segments of which I have ever 

 found upon a necked specimen. It would seem, therefore, that 

 the neck region appears when the supply of proglottids compris- 

 ing the earlier chain has become so much reduced that the devel- 

 oping reproductive organs begin to appear well into the region 

 occupied by proglottids which must have had an " anterior " origin 

 in the embryo, but the number of primary proglottids remaining 

 when the neck appears is quite inconstant. 



In the early part of my work, when I had not obtained so 

 complete a record of the conditions in the " necked " specimens, 

 I was inclined to question whether the segments formed at the 

 anterior end in the larval stage (Fig. 8, PI. IV.) were to be regarded 

 as proglottids in the same sense as those developed posteriorly, 



