ASPLANCHNA EBBESBORNII (rOTIFER) 411 



indistinct and finally is indiscernible. No vitellarium is formed 

 in the male, but instead the cells are differentiated into the testis 

 and the vas deferens, which are continuous with the urogenital 

 sinus, which in the male is ciliated. These cilia are very active 

 and aid in the passage of the sperm from the vas deferens into 

 the uterus. In the female the upper end of the reproductive 

 tube is very narrow and can be designated as the oviduct. The 

 urinary bladder is formed by an evagination of the urogenital 

 sinus. 



THE EXCRETORY SYSTEM 



The excretory system is ectodermal in origin. It arises from 

 special cells, which become separated from the outer embryonic 

 epithelial layer on its inner surface at the ventroposterior end 

 (figs. 77, 79, 87, and 90). These cells act hke teloblasts and, by 

 a rapid proliferation of cells in an anterior direction, produce on 

 either side of the ventral median line a mass of cells (text figs, 

 o and p) from which the excretory tubules are formed. The 

 cells of each band are arranged in definite rows. Later cavities 

 appear within the cell rows; these become continuous from cell 

 to cell and give rise to the lumen of the future forming tubules 

 (text figs, q and r). The cells directly concerned in this process 

 contribute to the tubule walls with a few persisting nuclei. 

 Later, each band becomes differentiated into a system of tubules 

 with nucleated walls (text fig. s). Text figure u represents the 

 condition of the embryonic excretory tubules of the right side of 

 the embryo, shortly before birth. Either half is composed of 

 one main and two recurrent tubules. The three tubules are 

 united at either end. The manner in which this union occurs 

 varies in different individuals. Each tubule has a distinct, non- 

 ciliated lumen with a nucleated wall. A number of small club- 

 like evaginations are formed on the main tubule of either side. 

 Their number and size vary. The maximum number found on 

 any one tubule was fifty. The free ends of many of these evagi- 

 nations are enlarged and contain cilia, which exhibit a constant 

 flickering motion. These club-shaped organs constitute the so- 

 called tags or flame cells. In their early formation the free ends. 



