FORMATION OF PROGLOTTIDS IN CROSSOBOTHRIUM. 203 



do even the oldest of them begin to show the sculpturing of the 

 posterior margin which is so characteristic for the adult. 



When these posterior proglottids are about 35 in number, there 

 begin to appear, in the unsegmented region just back of the 

 scolex, others which may be termed the "anterior proglottids." 

 These are produced in the reverse order, /. e., the youngest is the 

 most posterior one. 



From such a condition as this (Fig. 10, PI. IV.) the formation 

 of proglottids continues so that from this time on we may say 

 that the body is segmenting from both ends toward the middle. 

 This process gradually obliterates the unsegmented zone (Fig. 

 12, PI. V.) but specimens several centimeters in length may still 

 show such a " zone " at a point near the line dividing the anterior 

 and middle thirds of the body. 



When this zone finally becomes obliterated there have been 

 formed in the neighborhood of 50 " anterior" proglottids and 

 upwards of 400 "posterior" ones. With this obliteration of the 

 zone we reach what may be termed the fully formed adult condi- 

 tion as is shown by Fig. 14, a-n. After such a stage is readied 

 no more proglottids are formed, and the specimen remains un- 

 changed save for the continual maturing and separating off from 

 the posterior end of the motile proglottids. This latter process 

 finally reduces the total number of the segments to such an ex- 

 tent that the condition is perceptible at first glance. Such a speci- 

 men which had only 96 proglottids is shown in Fig. 16, a and b. 



When the number of units in the strobilla has been reduced 

 somewhat further, the region between the scolex and the first 

 proglottid begins an active growth and elongates into a neck 

 region as is shown by Figs. 21 and 15. After further elongation 

 and the continual separation of the terminal proglottids, this neck 

 region begins to form segments while there are still some of the 



o o o 



most anterior segments of the old chain remaining (Figs. 17 and 

 24). This new segmentation begins as does that previously de- 

 scribed by the appearance of posterior segments and later of an- 

 terior ones, and although I have no stages beyond the point 

 shown in Fig. 24, I think one is justified in the opinion that this 

 segmentation is the beginning of a new chain and that soon a 

 new strobilla will be formed which would be with difficulty, if at 



