48 ILLINOIS BIOLOGICAL MONOGRAPHS [336 



itself now about l.Smra. in length, begins to show faint transverse lines in its 

 anterior end, decreasing in intensity from ahead backwards. These are the 

 earhest traces of the divisions of the primary segments into the secondary seg- 

 ments which wall become the definitive joints of the anterior ends of the adult 

 strobilas. In other words the original primary larva, plerocercoid or strobila, 

 divides up into secondary strobilas which eventually separate from each other 

 and grow into the adult chains as described above for the species. But long 

 before separation takes place the entire development of the anterior segments 

 with their characteristic posterior auricular appendages and the formation in 

 particular of the first segment can be followed in these primary strobilas (Fig. 

 44). Whereas originally (Cooper 1914a :82, Figs. 5 and 6, PL V) attention 

 was drawn to young scohces with only 5 to 8 segments, it was found in connec- 

 tion with the present study that the latter number, about 8 in external view 

 or 16 or 17 in cleared specimens, is in reality that developed by the secondary 

 strobilas before detachment from the original chain. The smaller strobilas 

 are now looked upon as having been prematurely and accidentally separated 

 from the posterior end of the primary strobila. The attachment soon be- 

 comes very slight, owing to the rapid deepening of the constriction ahead 

 of the first segment, so that some time before the auricular appendages of the lat- 

 ter are fully delimited posteriorly very little manipulation of even alcoholic 

 specimens, let alone cleared ones, suffices to break up the chain. However, 

 there was found intact in the material at hand one primary strobila 88mm. in 

 length, showing twenty secondary strobilas, including the undifferentiated 

 anterior segments from which they are developed. Furthermore, the last 

 two of these, 10.4 and ll.Smni. in length, showed in their posterior unseg- 

 mented portions the earliest traces of the rudiments of the reproductive or- 

 gans. As has been already intimated the anterior segments form within the 

 secondary or definitive strobilas by a gradual demarcation from head back- 

 wards, first internally in the parenchyma — actually as transverse layers of 

 nuclei (Fig. 44) which will eventually form the posterior auricular appendages 

 — and then externally as shown in the figure. 



A continued search for evidence in connection with the question of whether 

 or not there is in the secondary strobila a definite number of segments (external 

 and genital combined) brought out further interesting facts. The number 

 was counted in several young strobilas, evidently not long separated from 

 the primary strobila, with the following results in the case of four typical speci- 

 mens: (1) Length, 19mm., number of segments, 45; (2) 27mm., 29 or 30; 

 (3) 26.5mm., 30 (the posterior ones here ripe as in the next specimen); (4) 

 41mm., 32 segments. It would seem from these data that there is a more or 

 less definite number of segments, which might be considered to be about 30. 

 But in No. 3 segments 9, 10 and 11 were not only much elongated but further 

 subdivided anteriorly; while in No. 4 similar conditions were present in seg- 

 ments 10, 11 and 12, excepting that in the case of the eleventh the aggregations 

 of nuclei indicating the subdivisions were in the posterior end. Similar elonga- 

 ted segments in still other strobilas show this condition in their middle regions. 



