EPITHELIUM OF TURBELLARIA 257 



The thickness of the epitheUum as well as the shape of the 

 nuclei is largely dependent on the state of expansion or con- 

 traction of the worm. Dorsally it averages l()/x. ventrally S.T/j. 

 in thickness.'^ 



Directly beneath the epithelium is the basement membrane. 

 This is in most places a well marked layer averaging 2.7 ij. in 

 thickness dorsally and 1.5/i ventrally/' but varying from 4/i to 

 a mere line from j^oint to point. Toward the edge of the body 

 it becomes very thin. Next to the epithelium the membrane 

 frequently shows a very distinct outline, on the inner side it is 

 less sharply differentiated from the parenchyma. It is evidently 

 differentiated chemically from both parenchyma and epithelium, 

 judging by the differerce in stain between it and these latter 

 tissues. In haematoxylin-eosin preparations, the latter are stained 

 light blue or gray, while the basement membrane is straw-col- 

 ored, being thereby very distinctly marked off from the other 

 tissues. In sections taken perpendicular to the surface, the 

 basement membrane appears nearly homogenous, but where the 

 sections are oblique or parallel to the surface a fibrillar struc- 

 ture is plainly visible. The course of the fibrillae, while more or 

 less irregular, is in general parallel to the surface and thus at 

 right angles to those of the epithelium. A continuity between 

 them and those of the parenchyma on the one hand, and the epi- 

 thelium on the other, I consider probable although I am unable 

 to demonstrate it positively. 



Jander ('97, p. 24) describes the origin of the basement mem- 

 brane as a ''Verdickung der Netzstrange bis zu deixi Maasse 

 die eine Basalmembran darstellt." The same 

 author (I.e., p. 27) describes the striations of the epithelium as 

 occasionally passing "durch die Basalmembran bis in die aussere 

 Langsmusculatur hinein," but qualifies this statement by adding 

 that "ist es auch nicht unmogiich irrtiimlicher Weise einen der- 

 artigen Zellplattenstreif in einen Bindegewebstrang zu verlang- 

 ern." This view is supported by my own observations of the 

 fibrillar nature of this membrane and its probable continuity with 



^ Average of seven measurements varying from 4 to 14;u. 

 ^ Average of seven measurements. 



