Intracellular canals in the skin of Phascolosoma. 193 



sacks are seen to be broken down, several sacks have become confluent; 

 the vacuoles in the protoplasm have joined with the confluent sacks 

 and in all probability the whole cell disintegrates. Some evidence 

 that this is the case is aflorded by the fact that in the same organ 

 in which these old cells with the huge sacks are found, there are also 

 found new young cells with small concentric sacks separated by a 

 broad space which is traversed by delicate filaments. In these young 

 cells no large vacuoles are present (Fig. 4). 



Besides these two extreme phases, there may be found in a single 

 organ a number of intermediate conditions. On account of these wide 

 diÖ"erences existing among the individual cells, it would seem that 

 the whole organ never becomes broken down at one time, for as old 

 cells disintegrate, it is probable that new cells are formed in the 

 upper or peripheral portion of the organ and that these cells are 

 pushed downward later as other new cells are formed above them. 

 Thus the oldest cells occupy the deepest parts of an organ, the 

 youngest the peripheral parts. 



The cytoplasm of these gland cells shows characteristic differences 

 corresponding with the size of the ampulla?. In the young cells 

 with small ampulke, the protoplasm of the entire cell shows a granular 

 condition while in the old cells the cytoplasm is without granules or 

 shows only a small granular area in the vicinity of the nucleus, the 

 rest of the cell possessing a coarse alveolar, non-granular structure 

 (Fig. 4). Again there is a marked difference in the staining properties 

 of the young and old cells. With the Biondi-Ehrlich triple mixture, 

 while the nuclei of all cells take the red stain, the cytoplasm of the 

 young cells shows a preponderance of the red color, that of the old 

 cells a preponderance of the green. The walls of the ampulke and 

 radial vesicles, the delicate filaments traversing the vesicles, the 

 sheaths of the canal and main duct always take the red color. 



The large nucleus lying within the sheath of the main duct is 

 constantly present. Regarding the cell body to which it belongs, 

 two interpretations are possible. According to one, the sheaths of the 

 main duct and its branches, including the radial vesicles surrounding 

 the ampullas, together constitute a cell of very irregular shape; a 

 cell which in form may be compared to a bunch of grapes with its 

 stem. The nucleus of the cell is the nucleus already described as 

 lying within the sheath near the point of union of the several canals. 

 According to this hypothesis, we have a single cell containing the 

 main duct, its branches and their terminal ampullae, and itself sending 



Zool. Jahrb. XIII. Abth. f. Morph. IQ 



