The Cilium. 195 



sarc. Whether or not the ultimate structure of the axil thread is 

 similar was a point not successfully determined, though it occasion- 

 ally aj)peared to be so." The axil thread is the apparently solid core 

 of the pseudopod. According to Calkins (Protozoa, p. 81), this 

 thread is composed of ''some unknown substance," which is "Proba- 

 bly stiffened protoplasm similar to the central axis of the reticulate 

 pseudopodia." Tig. 8, taken from Calkin's ''Protozoa," shows this 

 axil filament and the surrounding protoplasm according to his view. 

 My own work shows that the filament is really a bundle of fibrils 

 which are probably contractile. 



Structure. 



In this investigation Actinosphaerium eichornii was used. Ani- 

 mals killed in 2 per cent, osmic acid and studied in dilute glycerine 

 show a definite reticular network which forms the substance of the 

 trabecule that surround the large inter-trabecular spaces. (PL II, 

 Fig. 7.) The fibers of the trabeculae often unite to form larger 

 fibers, which in a few cases were traced to the axil filament of the 

 pseudopodia. 



That we are dealing with a reticulum and not alveoli, as held by 

 Biitschli, is evident from the following observations. The fibrillae 

 of the j)seudopods could in a few cases be traced into the reticulum 

 and evidently furnished some of its fibers. (PL II, Fig. 10). 

 Fibrillae could be found that were branching into smaller fibrils, and 

 in these cases alveoli were often found in the angles. (PL II, Fig. 

 9 ; PL III, Fig. 1.) Sometimes the angle was completely rounded out 

 by the alveolus. (PL II, Fig. 9, and PL III, Fig. 4.) 



In the course of this investigation an explanation was suggested 

 to me by Dr. Hodge which accounts for. all the observations of alveolar 

 protoplasm. It was this : If a viscid fluid bathing a reticulum would 

 tend to form alveoli in the meshes of the reticulum and round out 

 their angles, a perfect alveolar appearance would obtain. That such 

 alveoli do tend to form on the angles is seen in Fig. 1, PL III. 



Acting on this suggestion, experiments were made to determine to 

 what extent this would hold in oil foams. Silk thread was teased 

 into its idtimate fibers in an oil foam and mounted under a cover 



