291 LIFE HISTOR Y OF TREMA TODES—FA UST 29 



mucin glands, 4) The "Epithelzellen" of the monostomes and holostomes 

 (Figs. 37, 54) are characterized by large vesicular nuclei and vacuolated cyto- 

 plasm, similar to the "Blasenzellen" of Schwarze (1886) and Looss (1893). 

 They are modified parenchyma cells differing from the underlying layers not 

 in potency but in location. 5) As the secretory cells for the basement mem- 

 brane, these parenchyma cells have developed long acicular pseudopodia 

 toward the membrane and, in the larval holostomes, have penetrated into it. 

 All of these data point toward the parenchymatous origin of the basement 

 membrane. 



PARENCHYMA 



Soon after the fundaments of the digestive and nervous systems of the 

 cercaria are laid down, certain cells of mesodermal origin of the germ ball 

 become ovoid and are filled with milky white granules. These are cystogenous 

 cells, the "Stabenkornchen" of the German writers and the "cellules a 

 bS,tonnets" of the French. They develop most conmionly in monostomes, 

 amphistomes, and such distomes as form a heavy cyst. 



Other portions of the mesoderm are differentiated as the germinal epithe- 

 lium and the muscle layers. The remainder of the mesodermal cells is for a 

 considerable time potentially great, and remains undifferentiated (Looss, 

 1893:29). They constitute the parenchyma. Looss has compared these 

 cells of the mesoderm to the cambium of the plant. They are the "nicht- 

 veranderten Zellen," on the multiplication of which depends the growth of 

 the minute larva to the relatively large adult. As the animal grows the cells 

 of this region become more vesicular, vacuoles appear within the cytoplasm, 

 and acidophilous granules appear within the cell. The intercellular spaces 

 become more and more prominent. The cells are held together by bands of 

 ragged connective tissue which, for the most part, is the outgrowth of the 

 interstitial cells. Within this parenchyma complex there appear large tubular 

 lumina in certain definite regions, and, leading into these, tubes and smaller 

 tubules. These are the excretory tubes; at the ultimate ends of these are 

 found the capillaries and the flame cells (Looss, 1892:162; Thomas, 1883:116- 

 118). In the schistosome cercariae studied the main group of cilia is not at the 

 extreme ends of the ducts, but in a pocket in the posterolateral part of the 

 main trunks (Figs. 143, 145). It is of importance to emphasize here that 

 these excretory trunks and tubes are not lined by a wall of specialized cells, 

 but are merely lumina among certain cells of the parenchyma. It seems 

 highly probable that Looss 's view is correct as regards the flame or "Trichter, " 

 that it, too, is an intercellular lumen, into which the parenchyma-cell cilia 

 protrude, and that it is not in a hoUowed-out cell. The cilia are definitely 

 outgrowths of the single cell at the head of the capillary (Fig. 138), a cell 

 which is differentiated from the sister cells of the parenchyma by the possession 

 of a much smaller nucleus and densely granular protoplasm. 



