20Ô Robert Thompson Young,. 



seen at le. The wall of the capillaiy at this stage consists of fine 

 parenchj^ma strands and its lumen cannot be traced far bej^ond the 

 capillary cell. The observation of so fine a structure as the capil- 

 lary wall at this stage in its development, even with a magnification 

 of over 2000 diameters, is exceedingly difficult, and it is therefore im- 

 possible to say with absolute certainty whether the wall is a contin- 

 uous structure thruout its whole periphery, or is composed of inter- 

 secting parenchyma fibrillae. The latter condition, however, appears 

 the more probable. There is no evidence here to show that the 

 capillary cell is a sister to the flame cell shown. More of this anon, 

 under the discussion of Bugge's paper. 



The walls of the excretory ducts are composed of parenchyma 

 fibres and are continuous with similar parenchyma fibres on every 

 side. The lumina are formed as a result of a separation in the 

 fibres of the aggregated mass. 



At first the lumen of the ducts communicates with the sur- 

 rounding parenchyma spaces, their wall being composed of a net- 

 work of parenchyma fibres with intervening spaces thru which such 

 communication can exist. This is the condition to be found in the 

 posterior or embryonic end of the bladder thruout larval life and it 

 is by means of such communication that the excreted fluids may 

 reach the bladder cavity. Soon, however, after the development of 

 the ducts their walls deposit a thin cuticula on the inner surface. 

 This cuticula is always exceedingly thin, sometimes, indeed, so 

 much so as to be difficult of demonstration. 



The excretory ducts develop from innumerable centers and later 

 fuse to form the excretory plexus of the larva. There is no definite 

 sequence in their development, save that those in the anterior or 

 older end of the larva naturally develop before those in the posterior 

 or younger end. 



In Blochmann's Epithelfrage (1896, p. 10), is the following 

 statement regarding the flame cells and their development: "Dass 

 diese nicht durch Umwandlung einer Parenchymzelle entstehen, die 

 ihren Ausführgang nach den Hauptstämmen hin sendet, sondern dass 

 sie von diesen aus in das Parenchym einwachsen, läßt sich. ... in 

 der Wachstumsregion der Cestoden leicht erkennen. . . . Dass die 

 Wimperflammen etwas Besonderes sind, dass nicht eine beliebige 

 Parenchymzelle sich in eine Wimperflamme umwandeln kann, lässt 

 sich auch schon daraus erschliessen, dass sie sich durch Theilung 

 vermehren und dass sie nicht an Ort und Stelle entstehen, sondern 



