FORMATION OF PROGLOTTIDS IN CROSSOBOTHRIUM. 209 



of these "anterior" proglottids have appeared, Fig. 7 one in 

 which there are a few more and the differences between the pro- 

 glottid outlines is more apparent. These two figures moreover 

 illustrate the absence of a very definite relation between the 

 number of posterior segments formed and the time at which the 

 anterior ones begin to appear, for in Fig. 7, there are 56 posterior 

 and 10 anterior as against 66 posterior and in 6 anterior in Fig. 

 10. The marked differences in contour are better seen a little 

 later, when there are more anterior segments present. Fig. 8 

 illustrates this and shows that the oldest ones have already 

 assumed the shape so very characteristic for the adult proglottids 

 of this region, and even in those which are just appearing (Fig. 

 8) the suggestion of the four lappets which occur on the posterior 

 margin can already be seen. The contour of segments from this 

 region is seen again in the figures of PI. VI. and a stage which 

 is practically the adult is shown in Fig. 25, a, PI. VII. The four 

 lappets are very mobile in the living specimen and their appear- 

 ance differs considerably according to their state of contraction 

 and the angle from which the strobilla is viewed as will be ap- 

 parent from a glance at Figs. 14, <7, PI. V., 16, a, PI. VI. and 

 25, a, PL VII. 



The " posterior " segments have in their final condition four 

 flattened lappets (Figs. 14, n, PI. V., and 25, c, PI. VII.). This 

 sculpturing is distinctly different from that of the " anterior ' 

 segments until the latter come to occupy the terminal position 

 (Fig. 15, PI. VI.), when they assume the contours of typical pos- 

 terior proglottids. 



The formation of the posterior segments continues rapidly - 

 (Figs. 10, PI. IV., and 12, PI. V.} and this region presents an 

 appearance identical with that ordinarily met with in the cestode 

 strobilla. The anterior proglottids develop more slowly and their 

 total number is always much less than the posterior ones, hence, 

 the region where the two meet and where the non-segmented 

 zone is finally obliterated is well towards the anterior end. 



The total number of posterior segments can be determined only 

 approximately because there is always the chance that some have 

 been lost either under natural conditions or during the handling 

 incident to collection and preservation. Taking the maximum 



